I like TestDriven.NET (even though I use ReSharper) and I'm pretty happy with XUnit.net. It uses Facts instead of Tests which many people dislike but I like the difference in terminology. It's useful to think of a collection of automatically provable Facts about your software and see which ones you violate when you make a change.
Be aware that Visual Studio 2008 Professional (and above) now comes with integrated Unit Testing (it used to be available only with the Team System Editions) and may be suitable for your needs.
