If you are familiar with SourceForge, you might want to check out CodePlex. According to their website, CodePlex (www.codeplex.com) is "Microsoft's open source project hosting web site." It has a lot of the features that SourceForge provides (issue tracker, discussions, source code distribution, etc.), but has one really cool feature: it is based on Microsoft's Vistual Studio Team Foundation Server. This means that it can be tightly integrated into your Visual Studio development environment.
I'm no developer, but CodePlex seems like a really cool idea. In fact, a number of Microsoft project teams are actually participating in community development efforts using CodePlex. Check it out at www.CodePlex.com.