Not meeting more interesting people outside my discipline in college. My network is mostly limited to people in my field because I spent too much time with people similar to me because that was easy and comfortable.
If you are a software guy, especially a web guy? Making it known that you are willing to help all kinds of people with whatever they might want to do leads to all kinds of connections. I'm friends with music video directors, musicians, designers, artists, doctors, scientists, many of whom I met through doing a project for a friend of a friend.
