In an average work week I use:
Visual Studio 2008 Team Edition: The IDE of non-choice.
Reflector: There is no excuse for not having this if you work with anything that you haven't made yourself. And even then....
Resharper: Can't live without it. If you develop in VS2005/VS2008, professional version or higher, you should waste no time in getting this. This is a plugin which makes Visual Studio what it should have been in the first place. Microsoft is usually very slow to establish accommodation for sound development practices. And even Resharper is a wee slow for being the testrunner of choice.
NDepend: I love this tool. Have you ever wondered how to manage a large code base without actually having to read every single line of code? Then this is it. Also pretty straightforward to integrate into your build to enforce architectural constraints.
Redgate SQL toolbelt: We have a lot of stuff going around SQL Server. Even though I have database edition of Visual Studio, redgate are the ones that seem to actually have experience in the problems of managing such servers.
ANTS Profiler: Wanting to get to those hard-to-find and hard-to-reproduce performance issues? With very little overhead you can have it running. And, if you are able to reproduce it, you have it. Too many performance tools require you to predict exactly when you are going to see the problem. ANTS Profiler allows you to capture all, and then zoom in on it on the timeline.
* Mindmanager. (when you just want to get those thoughts down)
* Expresso. (regex helper)
* Editpad. (notepad on steroids)
* SPX from Moodysoft: (screen grabber with redlining and visual effects)
* Zoomit. Every presenter need this when sharing desktop with a projector.