If you have never heard of Resharper, then you should come out of your little cave right now! I have been using Resharper 1.5 for a long long while now, and it's incredible I have managed to work professionally without it. It's the best add-in I ever ran into (Visual Assist was my prior favorite).
Well, since a couple of months, I've been trying the Early Access Program for Resharper 2.0 for Visual Studio 2003. As of this writing, the latest build is 217. It is still rather unstable, but the productivity increase already outweighs the number of open issues. New builds are releases almost every week, so as soon as it gets stable, I'm sure I'm going to get myself a license.
Also, a couple of collegues have tried it with Visual Studio 2005 and were surprised how many productivity features Microsoft forgot to built-in. Even with the enhanced VS 2005 editor, Resharper is still a major improvement.