Scott Gutherie the Program Manager for many .NET technologies at Microsoft including IIS and Visual Studio announces the release of the .NET framework source code.

Whilst it's not quite open source it certainly helps to see how production quality code is written.  It's .NET v3.5 and VS2008 only but it's certainly a start.

Check it out. http://weblogs.asp.net/scottgu/archive/2007/10/03/releasing-the-source-code-for-the-net-framework-libraries.aspx