Resharper
I've been using Resharper for a few weeks now and I'm impressed with the productivity gains I've experienced in Visual Studio. They continue to make bug fixes and feature enhancements on a regular basis. It's free to try the evaluation for now and I'm definitely going to buy it when it's released.
My favorite features are:
- Better syntax checking (akin to VB.NET) for C#. You don't need to compile before finding out you have errors
- Better code highlighting and code formatting
- Simple refactoring support (rename) which I do all the time.