Just recently, Microsoft opened up a new community development Web site named CodePlex to start and share development projects with others. It includes projects such as IronPython, which is the implementation of the Python programming language on .Net and the “Atlas“ Control Toolkit, which is a collection of web-client components built upon the ASP.NET “Atlas” Framework.
Now Google is offering a site of their own named Google Code. I briefly looked through the CSharp section and saw the Pandora Player, which looks interesting. For those who don't know what it is, Pandora.com is a music project that helps you discover other music similar to those that you already like. The Pandora Player simply displays the Flash-based player in an application window that can be minimized to the system tray.
All these new places to host open-source projects are great, but don't forget about SourceForge.net, which is home to many great applications as well. I just went there the other day to get log4net, which is a logging framework for .NET. Perhaps that wasn't the best example since they have officially moved now to http://logging.apache.org/log4net. Oh well, you'll find lots of other nice stuff there still anyways.