All .NET developers, proceed immediately to Lutz Roeder's site (remember, I have his office shelves) and download Reflector 5.0.  It has a ton of new features.  So many that I won't attempt to list them here.

This tool was a part of my programmer's arsenal before I came to MS, and I still use it daily to spelunk around the managed code in the CLR and framework, even though I have access to the source code. It is an incredibly powerful tool that combines static analysis, documentation, disassembly (into multiple languages), search, etc.

If you're looking to get into C# for the first time (and I know several of you are), it is helpful to be able to look at your code as if it has been written in C#, or look at code that was written in C# in another language so you can see how C#'s constructs are saving you time (over some languages).

Ok, I said I wasn't going to list features, but I know some of you won't click through the powerpoint deck.  The new version can fold anonymous delegates, and even C# 3.0 lambdas and LINQ queries back into their original syntax.  Of course it's configurable, so you can still see what the compiler's doing for you.  It also adds proper Vista support.  It also adds support for code:// urls (Ex. code://mscorlib/System.Object). Oh, and the Add-Ins are hosted on CodePlex now.  Tons more stuff.  Get it now. No excuses.