There are a number of them. NUnit is a system that we like. It uses .NET debugging support seamlessly and is painless to use. You have no excuse not to write unit tests anymore!

Contributed from George Shepherd's Windows Forms FAQ,

Visual Studio 2005 Team System includes unit testing, see the whitepaper Visual Studio 2005 Team System: Enabling Better Software Through Better Testing in the MSDN Library.

Stuart Celarier, Fern Creek