Not that you haven't had it before but now it's better...
if you are working with .net in any way you certainly heard about this announcement ScottGu just did (see http://weblogs.asp.net/scottgu/archive/2007/10/03/releasing-the-source-code-for-the-net-framework-libraries.aspx)
On the one hand this is good news and the screenshots show how it looks like. The ability to debug into base libraries is certainly a real benefit.
On the other hand if you really wanted to know what's under the hood you already had the chance to do so. First and foremost using Reflector (by Lutz Roeder, see http://www.aisto.com/roeder/dotnet/) and secondly with the Shared Source version of the runtime (the artist formerly known as Rotor, see http://www.microsoft.com/downloads/details.aspx?FamilyId=8C09FD61-3F26-4555-AE17-3121B4F51D4D&displaylang=en)
So, not that I want to take the hype of this news...but it is not that you haven't had it before. Now it's better.