Hack-a-thons are a great way for companies like Facebook or Twitter to get their developers together to find the next great feature. For Y Combinator, it's a way to find the next developers to add to its incubator. For the developers, it's a place to build the future via apps and friendships. Y Combinator is in an unremarkable commercial park located in Mountain View. Nestled in the middle of buildings erected in the late 1970s to early 1980s, an orange Y and the meandering developers clutching notebooks (mostly MacBook Pros) are the only signs alerting you that behind the dark...