Microsoft has a community site for code called CodePlex. Like SourceForge, you can create your own projects and download code from existing ones. The good think is that it does not require Microsoft Passport to register / sign in (unlike GotDotNet). However, it is not categorised - i.e. broken down by type (database, desktop, finance, games etc), programming language (C#, VB.NET) or interface (WinForms, WebForms).
Despite that, it is a lot easier to navigate than SourceForge and contains several useful projects (IronPython, Jad Engine - C# + MDX 3D Game Engine, "Atlas" Control Toolkit and more). It also allow GPL'd projects (which I don't think GotDotNet does) - Jad is under the LGPL.