We use the Team Foundation Unit Testing tools which, without any explanation, ties extremely well into Visual Studio and works very well! Depending upon your needs this one tool may not be enough. There are other tools to help with the testing of the UI portions such as how the end-user interfaces with the application and then how that information which is relayed back to the developer in a more accurate format. In this case, Morae from Tech Smith (the creators of Snagit and Camtasia) have a tool specifically for this type of need (http://www.techsmith.com/morae.asp#). There are other unit testing tools out there such as Parasoft, which I have not used but have heard good things.
So it really just comes down to your needs. But you definitley cannot go wrong with Team Foundation.
Check out also http://www.mbunit.com/. It does more compared to NUnit.