21. Teespring

Founders: Walker Williams, Evan Stites-Clayton
Launched: 2012
Funding: $58.1 million
Valuation: N/A
Disrupting: Retail, apparel, manufacturing
Rival: Etsy

If you've ever seen someone wearing a T-shirt with a really clever and unique slogan, chances are, Teespring made it. The San Francisco-based start-up allows anyone with a design idea to create and sell custom T-shirts. Its staff of about 300 employees manufactures the custom-designed shirts, manages online sales for these fledgling designers, collects payments, ship the clothes and handles customer service. Teespring charges designers about $9 a shirt, depending on the fabric and complexity of the design. The designer decides on the retail price they want to charge and keeps the difference.

Read MoreFULL LIST: 2016 DISRUPTOR 50

Walker Williams and Evan Stites-Clayton came up with the idea for the company in 2011 when they were seniors at Brown University in Providence, Rhode Island. The pair wanted to make a T-shirt to mark the closing of a favorite nearby bar but were shocked at the price and the need to guess in advance how many shirts they should order. Instead, Williams built a website to take orders and advertised it on Facebook. When he got a great response, he felt certain there was a business there.

So far, he's right. Teespring claims to have shipped 15 million T-shirts around the globe, more than double the number from just two years ago. It says about 20 of the designers that use its service make more than $1 million a year in sales from their T-shirts. Venture capital heavyweights, such as Andreessen Horowitz and Khosla Ventures, have already invested $58 million in the company.