I think I can safely assume that everyone knows about SourceForge.net, where first-class .NET projects such as NUnit, NAnt, NProf as well as many others are hosted. Much less known is GForge, which is a fork that you can host yourself based on the GPL version of SourceForge before it was made closed-source by VA Linux in 2001.
Since that time GForge was improved, and it has reached version 4 just recently. To give you a really high-level idea of what it is, a short quote from the project page itself: GForge has tools to help your team collaborate, like message forums and mailing lists; tools to create and control access to Source Code Management repositories like CVS and Subversion. GForge automatically creates a repository and controls access to it depending on the role settings of the project.
Tool-wise, this package can come in handy when you want to get started on a development process in your company, covering the entire lifecycle. For free.