Update: renamed project to YUnit to avoid clashing with other frameworks.
I've been playing with custom test types for team test lately and the result of this experiment is YUnit. A microscopic test framework that lets you write tests anywhere since it only uses the Conditional attribute. That's right, any public static parameterless method tagged with [Conditional("TEST")] becomes a test :)
If you've always dreamed of implementing your own custom test type, this sample could be helpful to you. Remember that this is only a sample and comes with no guarantees of support.
Sources and installer available at: http://code.msdn.microsoft.com/yunit.