Well in concordance with this, I've decided to post about my favorite tool. Well in coming up with my favorite tool, I had to search through many of them that I use and love. But most recently, I believe my favorite tool, that I use every single day has to be ReSharper from JetBrains. This add in to Visual Studio (I use Resharper 2.5 for VS2005) adds so many tiny benefits it would be hard to list them all. Just go their site and look through the feature list. It's simple, fast (as of the new version, the past versions did have some speed issues) and it stays out of the way. I've gotten to the point now where I don't even realize it is there. I was at a coworker's machine, which does not have Resharper installed, and I was amazed that I was able to get anything done without it.
Hard to say what my favorite feature is, I really like the Usages (Alt-F7). The Ctrl-Click Go to Definition is a huge time saver. Surround With (Ctrl-Alt-J) is another tool I use alot. I could go on and on. The hardest part for me has been learning some new keystrokes, for some of the lesser used items, the keystrokes still escape me from time to time, but I'm getting better.
I won't get into a ReSharper vs Refactor Pro holy war, as both products have a ton of nice features. I wish I could take the best of both, combine then into one and you would have one killer app.
Oh BTW, JetBrains has just put out an update as I've mentioned previously