Q: Finding ASP.NET source code Where can I find source code for e.g. XmlSiteMapProvider and other "built-in" stuff for ASP.NET (3.5)?

A: You can use Reflector. It's free, portable and fast.

A: I'm not sure whether ASP.NET is included, but some parts of the .NET framework are linked to Source Server - you can then step into the framework code, almost as if it were your own. Here's an article to get you started:
http://blogs.msdn.com/sburke/archive/2008/01/16/configuring-visual-studio-to-debug-net-framework-source-code.aspx

A: You can never find the source code of inbuilt controls due to the fact that they are compiled into assemblies that are part framework.
Why do you need the source?
