If you practice TDD in any fashion you are going to want to get NCrunch. It is still in beta, and may become a commercial product at some point but it is pretty stable and for me has become an essential tool. Imagine being able to just write code (test or functional) without having to pause to run tests. Imagine making a change to existing code and knowing immediately if it broke any tests without having to manually run the tests. This continuous development ability is what NCrunch delivers and it has made me significantly more productive in my coding and in a couple of weeks I found this tool as necessary as having a productivity tool like CodeRush or Resharper.
Just spend a few minutes watching the video on the NCrunch home page and if you aren’t immediately hooked on the idea I will be very surprised.