Startup idea: open source equivalent of an Internet dating site
GitHub and friends are pretty great, but they only help when you have code to share. In my case, I had an idea that I wanted to discuss with other people before I started writing code. Also, I think it would be easier to find partners if they can have their say in the design, before half of the code is written.
More discussion on stack exchange: http://programmers.stackexchange.com/questions/154022/how-do-you-find-partners-for-open-source-projects