The NUnit team has released the first beta of the fourth major release of NUnit. I've been playing around with the source tip of this version for a couple of weeks and it is cool:

• Assert.AreEqual now supports comparing arrays
• CategoryAttribute allows you to group tests into logical categories like "FastTests" or "SlowTests"
• ExplicitAttribute allows you to say that a test should only be run when explicitly indicated by the user
• NUnit GUI now has checkboxes so you can select specific tests to run
• Comes with a built-in, lightweight Mock Object facility
• Mono support