I found my cofounder (or more accurately, he found me) via weekly video game sessions in college. Since our startup deals with games, it seemed like a natural fit.

More broadly speaking, I suggest starting with "what kind of person do I need?" and go from there. For example, if your idea is to build a social networking site for environmentalists, I would see if I had any friends studying Environmental Science. Or if you're a business guy in need of a coder, I would hang around the Computer Science department.