Mark Riffey pointed out Might Moose to me - you can find it at http://continuoustests.com/. This is a continuous testing tool for .NET that executes unit tests while you code so you have continual feedback on how your changes are affecting your code base. From the web site:
ContinuousTests is a continuous testing tool for the .Net and Mono platform. It helps you with the way you do TDD by taking care of all compiling and testing in the background to help you work more efficiently. It also figures out exactly which tests to run, based on the changes you just made. So, instead of running all tests, it will only run those affected by your changes.
In addition to making sure your work flow is optimized, its features include graphs visualizing the relation between a piece of code and the tests run on it, annotation inside your code showing risk you are taking changing this piece of code, and more.
Mighty Moose is (as near as I can tell) a free product and is available as a Visual Studio extension or as a standalone product.
RedGate .NET Demon
RedGate has a similar product called .NET Demon. As with Mighty Moose, your code is compiled as you type, and only those unit tests affected by your changes are run.
.NET Demon is a commercial product with an introductory price of $40.
There's also NCrunch, another freely available Visual Studio add-on. From the home page:
NCrunch is an automated parallel continuous testing tool for Visual Studio .NET. It intelligently takes responsibility for running automated tests so that you don't have to, and it gives you a huge amount of useful information about your tests (such as code coverage and performance metrics) inline in your IDE while you work.