Location: San Francisco
What it is: A mobile payments company
Date founded: 2009
Company size: 200 employees
Financing: $341 million from Khosla Ventures, Greg Yaitanes, Marissa Mayer, Dennis Crowley, Kevin Rose, First Round Capital, Ron Conway, Biz Stone, Joshua Schachter, Shawn Fanning, Zachary Bogue, Andrew Rasiej, David Lee, Esther Dyson, robin Chan, Fritz Lanman, Brian Pokorny, Xavier Niel, Jim Pitkow, Sequoia Capital, Visa, KPCB, Tiger Globla, Richard Branson, Starbucks, CrunchFund, Citi Ventures, Rizvi Traverse Management
Why it's a good bet: You might not get a ton of equity if you join Square now, since it has raised hundreds of millions of dollars and has hundreds of employees. But Square is undoubtedly one of the most promising startups right now. Jack Dorsey, its CEO, co-founded Twitter and it's onboarding small businesses quickly.
The company also just released a new way to send cash via email, proving that you the company will keep innovating in the mobile payments space.