The phrase “side hustle” seems to have become a bit of a buzzword lately, as the gig economy has made it easier for us to adopt multiple money-making strategies alongside the 9-5. Yet in some cases, a simple side project that started out as a way to make extra cash can turn into an extremely successful business. Click or scroll through to see the 15 most impressive side hustles that turned into multimillion- and even billion-dollar companies.

Probably the most successful side project of all time, Apple started out as Steve Jobs and Steve Wozniak's extra-curricular hobby. In fact, Wozniak has admitted he developed the first Apple computer in the early 1970s in his cubicle at Hewlett-Packard, where he worked at the time. Jobs was working for Atari in 1973 when he teamed up with Wozniak to refine the design of the pioneering computer. The first prototype was built in 1975. A year later, the pair founded Apple Computer and resigned from their jobs.

The first Apple computer flew off the shelves and Wozniak followed it up with the Apple II, the first PC to display colour graphics, which went on to become one of the world's first highly successful mass-market microcomputers. Today, Apple is worth a dizzying $1.3 trillion (£1tn), making it the biggest public company on the planet.

Now check out the American brands that have fought their way back from the brink

In 1995, former IBM programmer Craig Newmark was working for investor Charles Schwab in San Francisco when he created his eponymous listing site. It started out as a simple email distribution list to his friends, letting them know about events in the Bay Area. But as it was the early days of the internet, the list caught on and subscribers began posting classified listings for jobs, lonely hearts and more. By 1999, Newmark quit his job to focus on the rapidly-growing site and had hired nine employees by the following year.

The website expanded into scores of US cities in the early 2000s, and became a household name during this time. Craigslist now covers 700 cities across 70 countries worldwide and the website, which makes its money charging for some jobs and real estate listings, generates revenues of more than $1 billion (£777m) a year.

In 1995, 23-year-old University of Maryland footballer Kevin Plank found himself feeling frustrated by his thick, heavy football kit, so created his own moisture-wicking T-shirt that would keep him cool and dry on the field. Friends caught wind of his bright idea and he started making shirts for them too, which helped him to perfect the design. He maxxed out his credit cards to raise $40,000 in funds to launch the clothing line, founding Under Armour in September 1996, which he started manufacturing out of his grandmother’s basement.

His big break came when he made his first large kit sale to Georgia Tech’s football team, to the tune of $17,000. After that, a number of NFL teams began purchasing his clothing and by his second year in business, he had sold $100,000 worth of the garments. It peaked in 2015 when stock prices reached $104.10 (£81) per share. Now the company looks to be in trouble, reporting a loss in the first quarter of 2017 and having had to lay off 680 employees in the past couple of years. In October 2019, it was reported that Plank would be stepping down from the position of CEO.

The go-to destination for unique handmade products, Etsy was founded in a Brooklyn apartment in 2005 by three friends: Rob Kalin, Chris Maguire and Haim Schoppik. The trio wanted to create a platform where makers could sell their items, and in the space of just two years, Etsy had almost 450,000 registered sellers, generating $26 million in annual sales. 

Yet Maguire and Schoppik became exasperated with Kalin (pictured), who had a reputation for being a bit of a daydreamer, and in August 2008 they both left the company. At that point, Kalin hired former Yahoo employee Chad Dickerson as Chief Technology Officer who ended up replacing Kalin as CEO just three years later. The company went public in 2015 and is currently worth $6 billion (£4.7bn).

Jack Dorsey was working as a web designer at a podcast company called Odeo when he started designing Twitter – yet his original vision for the site was quite different to what it ultimately became. He intended for it to be simply a platform to chat to his friends on a desktop. But when Odeo started to struggle, employees were advised to work on their side projects and during a hackathon in March 2006, Dorsey built an early version of the platform.

Odeo morphed into Twitter, taking the majority of the former company’s assets. Things moved pretty quickly after that. In March 2007, Twitter was named best startup at South by Southwest media festival, then during its first funding round that summer it raised $5 million and was valued at $20 million. At the time of writing, Twitter is worth $26.89 billion (£20.9bn).

In 2007, Joe Gebbia an Brian Chesky stated renting out the floor space in their San Francisco apartment to earn some extra cash, letting visitors stay on air beds – hence the name Airbnb. Joining forces with their former roommate Nathan Blecharczyk, in August 2008 they launched the company. It had a few stumbles at first and the trio struggled to get it off the ground, but in 2009 they joined a Y Combinator which helped them to raise $20,000.

Then they realised that poor quality photos were stopping people from being interested in listings, so they went door-to-door in New York taking photographs of the properties. Things began to look up and in April 2009 the company gained a $600,000 seed investment from Sequioa Capital. By 2011, the company was in 89 countries and was valued at $1 billion (£777m). In 2018, a Forbes estimate valued the company at a whopping $31 billion (£24bn).

Read more about Airbnb and how it became a billion-dollar company

Andrew Mason was a student at the University of Chicago when he launched The Point in 2007, an online platform designed to help people accomplish their fundraising goals. But it was Groupon, the site that it inspired, that ended up being successful. Launched in November 2008, Groupon allowed local businesses to advertise deals in order to attract customers.

By the end of 2009, Groupon had spread to 28 US cities and it showed no sign of slowing down. By the time of its IPO in June 2011, Groupon was worth $16 billion (£12.4bn) – although the company may have peaked at that time. Nowadays, the business is worth $793.68 million (£616.6m) and its stock price is falling fast. The company says its turnaround strategy will involve shifting away from deals and towards local experiences. 

In 2008, unlikely fashion mogul Julie Deane started making leather satchels from her kitchen table in Cambridge, England as a handy sideline to help pay to send her daughter, who was being bullied, to a private school. Deane didn't expect to make much money selling the bags and was astonished by the response. Within a year, the colourful satchels were gracing the pages of Italian Vogue and had become the latest must-have for fashionistas. 

The orders flooding in, the firm was soon turning over millions and the brand has fast become a classic. Deane was rewarded in 2014 with an OBE for services to entrepreneurship. However, in the past four years the company has suffered losses, reporting a loss of $1.8 million (£1.4m) between June 2018 and June 2019.

Now discover the 15 genius inventors who missed out on millions

In the space of 11 years, Houzz has established itself as one of the world's top interior design and home improvement platforms, and its super-popular website, online community and apps have become indispensable to homeowners and professionals alike. Fittingly for a platform that encourages users to undertake their own pet projects, Houzz began life in 2009 as a side hustle for husband-and-wife team Alon Cohen and Adi Tadarko, who set up the website from their home in Palo Alto, California.

The couple created the site to help them find suitable designers and tradespeople for their home revamp project. Through word of mouth their resource soon garnered a sizeable fan base and was founded as a company in 2010, before going on to conquer the world in the 2010s. The site now has a community of 40 million homeowners. At its last funding round in 2017, the business was valued at $4 billion (£3.1bn), although the company laid off 180 staff in January 2019 and is thought to be preparing for an IPO.

Believe it or not, Instagram was initially a side project and a mere feature of a more complicated app called Burbn. Ex-Google employee Kevin Systrom (pictured right) came up with the idea of a check-in app linked to social games while working at start-up travel recommendation website Nextstop in early 2010. Systrom quit his full-time job later that year and teamed up with fellow Stanford Univeristy grad Mike Krieger (left) to build Burbn. At first, the app was packed with features – photo sharing was only one of its many functions.

Keen not to overcomplicate the app, the pair decided to focus entirely on photo sharing and the rest is history. Instagram, which launched in October 2010, attracted 10 million users within a year and was acquired by Facebook in 2012 for $1 billion (£777m), making its founders very rich indeed. In June 2018, Bloomberg analysts estimated that the app was worth more than $100 billion (£77.7bn).

WeWork has been mired in controversy lately, but the startup’s history remains impressive. The shared workspace business laid down its roots in 2008 when two coworkers, Adam Neumann and Miguel McKelvey, initially launched a company called Green Desk which focused on sustainable workspaces. Yet the pair quickly realised that the community aspect of the company had attracted customers, so they sold Green Desk and used $300,000 of investment to create WeWork in 2010.

In January 2019, WeWork announced it would be rebranding itself as We Company. Despite losses of nearly $2 billion (£1.6bn) in 2018, the company was valued at  $47 billion (£36.5bn) ahead of its IPO planned for October that year. However, after the company made a S-1 filling to the SEC major holes in the profitability of the business model as well as the management style of CEO Adam Neumann (pictured) were revealed, and the valuation was reduced by 80% and the IPO was postponed. But things could be looking up: Adam Neumann stepped down as CEO in September 2019, and backer SoftBank is reportedly taking over the company.

Now read about other companies facing make-or-break in 2020

In 2011, Maine carpenter Paul Lancisi began crafting baseball bats as a side gig, putting his years of carpentry experience and skills to good use. In no time at all, the talented sports fan was churning out premium, much sought-after bats. The Major League Baseball (MLB) soon caught wind of Lancisi's exceptional bats and added the carpenter to its prized list of suppliers, one of only 33.

Lancisi now devotes 100% of his time to making the bats and his business now turns over millions of dollars a year. The enterprise is positively thriving, pumping out 35,000 baseball bats a year. At present, the company employs 12 people in Maine and three in Arizona, and Lancisi is building a new production facility and hitting facility in Shirley, Maine, where customers can try out the bats.

Teen gamer Palmer Luckey was hanging out on Meant to be Seen (MTBS)'s virtual reality (VR) and 3D discussion forums in early 2011 when he came up with the idea for an affordable virtual reality headset. That same year, the enterprising 18-year-old cobbled together a basic prototype of the Oculus Rift VR device in his parents' garage in Long Beach, California. Fellow MTBS forum regular and games developer John Carmack saw the design and showcased the prototype at the E3 convention in 2012.

A massively successful Kickstarter campaign followed, raising $2.5 million (£1.9m), and Luckey's VR company was eventually bought by Facebook in March 2014 for a whopping $2 billion (£1.6bn), a move he described as "the best thing that's ever happened to the VR industry". Luckey's success hasn't been without controversy, however, as the Oculus founder was fired by Facebook in 2017. Luckey has since told CNBC that it was "for no reason at all" but that his $10,000 donation to a pro-Trump group could be "something to do with it".

The co-founder of Ludicorp, the firm that brought us Flickr, Stewart Butterfield created a company called Tiny Speck in 2009 in order to develop Glitch, an online social role-playing game that he thought was destined for great things. The game launched in September 2011 after a delay of six months, but due to unforeseen glitches – you couldn't make this up – the aptly-named game was 'unlaunched' not long after and officially abandoned in November 2012.

Luckily, Butterfield and his colleagues had developed a real-time collaboration app on the side called Slack as a way of communicating among themselves, and decided to bring it to market. A surprise hit, Slack was launched in 2013, swiftly adopted by countless businesses worldwide and until 2018, it was the fastest company ever to reach a billion-dollar valuation. Now worth a massive $14.65 billion (£11.4bn), some commentators are speculating that 2020 will be the year Slack is acquired.

When Archie Hewlett decided not to go to university he started to work in recruitment. However, after making his own loafers when he couldn't find an affordable design that he liked, his friends started to pay him to make them shoes and he spotted a business opportunity. Aged 19, he borrowed $6,435 (£5k) from his parents and launched Duke & Dexter in 2014.

Six years later, the brand has its own flagship store in London's Covent Garden and was set to turn over $3.2 million (£2.5m) in 2017. Duke & Dexter's success has been boosted by its many celebrity fans, including Snoop Dogg and actor Eddie Redmayne, who wore a pair of Duke & Dexter shoes to collect his Oscar in 2015, pictured.

Read more about teenagers who got rich all by themselves