An implementation of Kent Beck's famous XUnit TestingFramework designed specifically for unit testing components written for the MicrosoftDotNet platform.
v0.8.5 beta2 was just released and is available for download at: http://sourceforge.net/projects/dotnetunit/
See also NunitFramework.
Please note that these are two separate projects with differing approaches to creating UnitTests, so please be sure to check them both out.
Consider also HarnessIt, a vendor tool. (But it won't be free.)
CategoryDotNet CategoryTesting: TestingFramework for RegressionTesting of (MicrosoftDotNet) components and applications.