If you've ever wanted to step into the .NET framework library classes as you're debugging your project then you'll be happy to know that Microsoft has released the .NET Framework library source code.  Step in and out of various framework classes, ASP.NET controls, plus more.  Read more about it on Scott Guthrie's blog:

http://weblogs.asp.net/scottgu/archive/2008/01/16/net-framework-library-source-code-now-available.aspx