Are you thinking of freelance professionally, as a hobby or something to bring in more income?
SourceForge would be a good source if you wanted to find some work to do as a hobby, but not to earn income (unless you wanted to offer support/customization/value add to open source projects).
I worked for a few years as an independent contractor. About 1/3 the jobs I found searching various online job post sites, another 1/3 came from local headhunters and the rest came from people I knew in the industry that I'd pester with emails letting them know I'm available.
I have a friend who has decided to be a kind of wandering programmer. He works for a few days at one site, packs up and moves to the next town, only staying a few days before the next. He meets all his contacts via Facebook, MySpace, Twitter, etc. and blogging about software development. Networking is a powerful tool these days. If you want to freelance professionally, your network will be what gets you work.