My preferable unit testing framework is NUnit (as for the most of the people around, is not it?). It is from a family of xUnit. The framework is easy to use for either simple tests or more complex test suites.
What is my pros?
#1 It is actively developed and supported (its current stable version is 2.5.10)
#2 It has a good documentation
#3 It is an open source
#4 Written in C# entirely
#5 Visual Studio and Resharper support GUI of the unit test to run them easily from the IDE
#6 Also the framework is shipped with a bunch of utilities, like a console runner, a gui runner and more else
If you have not yet you can visit its website to try to do your own finding – NUnit