In response to chrisg's list of a few of his favorite things I thought I'd list some of mine.
Now, I wouldn't immediately think of these as my *favorite* things (except for Reflector), but they come in very handy when trouble-shooting those tricky issues, that you just have to love em.
Fiddler HTTP Debugger (IE plug-in)
http://www.fiddlertool.com/fiddler/
Lutz Roeder's .NET Reflector - excellent tool for answering those questions about the internals of the framework, or recovering lost source-code:
http://www.aisto.com/roeder/dotnet/
Microsoft Network Monitor 3.1 - new interface and you can customize the protocol analysers
http://www.microsoft.com/downloads/details.aspx?familyid=18b1d59d-f4d8-4213-8d17-2f6dde7d7aac
.NET CLR Profiler for .NET 1.1
http://www.microsoft.com/downloads/details.aspx?familyid=86ce6052-d7f4-4aeb-9b7a-94635beebdda&displaylang=en
.NET CLR Profiler for .NET 2.0
http://www.microsoft.com/downloads/details.aspx?familyid=A362781C-3870-43BE-8926-862B40AA0CD0&displaylang=en
Managed Stack Explorer
http://msdn2.microsoft.com/en-us/vstudio/aa718348.aspx
And an unrelated, but useful tool for blog posts, Windows Live Writer
http://writer.live.com
And let's not forget the code formatting plug-in for posting code snippets:
http://dunnhq.com/codeformatterforwindowslivewriter