Y Combinator is a startup accelerator based in Mountain View, CA.
In 2005, Y Combinator developed a new model of startup funding. Twice a year they invest a small amount of money ($120K) in a large number of startups (most recently 68). The startups move to Silicon Valley for 3 months. The YC partners work closely with each company to get them into the best possible shape and refine their pitch to investors. Each cycle culminates in Demo Day, when the startups present their business plans to a carefully selected, audience of investors.