Codeplex
I've started a project on Codeplex and I'm very impressed with how easy Team Foundation Server is to use compared to Rational ClearCase and ClearQuest. It integrates seamlessly with VS2005. I'm still getting used to it, but running into some integration issues between VS C# Express (required for XNA) and Team Foundation Server. I'm currently using VS2005 Team Edition, VS Express, and the command line (tf) in my workflow.