Sometimes, what started as a part-time job on the side becomes bigger than a person’s primary job. This has happened to a few entrepreneurs and celebrities.

In trying to solve a problem or seizing an opportunity, they saw their so-called side hustles explode, increasing their fortunes in the process. And, not all of them quit their primary jobs after something like this happened.

Here are eight inventors and creators whose side hustles took off and made them a lot of money.

1. Stewart Butterfield’s Slack

Butterfield must have a natural knack for tech. He really wanted to develop video game platforms but kept getting richer by his lucrative side hustles like Flickr, which he sold to Yahoo for $35 million.

Slack? A messaging platform he made for his employees.

He’s now worth $2.8 billion and preparing to take Slack public.

2. Michael Jordan’s Nike Air Jordans

Michael Jordan became a billionaire in 2015 from his shoe deal with Nike.

He’s enjoyed some lucrative corporate sponsorships as a professional athlete and the Charlotte Hornets contribute to his wealth considerably too.

3. Sara Blakely’s Spanx

Blakely sold fax machines for Danka when she developed Spanx. But even after Oprah put her coveted seal of approval on them, Blakely still held onto her day job while she wrangled Spanx orders out of her garage at night.

She didn’t quit until Neiman Marcus and Saks Fifth Avenue started selling them.

Now she’s estimated to be worth $1 billion.

4. Mark Benioff’s Salesforce

Benioff was working for Oracle when he came up with the idea that eventually turned into Salesforce.

His salary as a vice president at Oracle was $300,000.

He quit so he could make Saleforce a major competitor with Oracle and now he is worth $6.7 billion.

5. Rihanna’s Fenty

Rihanna has made millions making hit songs. But it’s her beauty and fashion businesses that have made her an absolute fortune.

She is the creative director of Puma and has her own beauty and fashion lines with French luxury powerhouse LMVH.

Her beauty line is worth $3 billion and with her 15 percent ownership, her net worth is $600 million.

6. Andy Dunn’s Bonobos

Dunn and his roommate Brian Spaly sold men’s khakis designed by Spaly while both were full-time students at Stanford’s Graduate School of Business.

Their car-trunk business turned into Bonobos. Spaly left the company and Dunn sold it to Walmart for $310 million.

Dunn is now Walmart’s senior vice president of digital brands.

 

View this post on Instagram

 

A post shared by Bonobos (@bonobos)

7. Jay-Z’s Tidal and D’Ussé

Hip hop mogul Jay Z made hundreds of millions of dollars with his music, but it’s his ventures with music streaming and his own cognac that has put him in billionaire territory.

 

View this post on Instagram

 

A post shared by JAY-Z (@jayzz_official)

8. Ashton Kutcher’s Sound Ventures

Kutcher has made millions with his movies and role in TV’s Two-and-a-Half-Men, but as it turns out, he’s a brilliant investor.

Kutcher and his talent agent, Guy Oseary, built up a portfolio that in 2016 was valued at $250 million according to Forbes.

Their strategy was to invest in start-ups like AirBnB, Uber and Skype.

 

View this post on Instagram

 

A post shared by @_ashton_kutcher_