MbUnit, come one, come all
Something that I have never understood is why MbUnit is so over looked. MbUnit supports the same assert set as NUnit, so much so you can flip referances and it works with out fault. MbUnit then add's more assert tests and can be extended just like NUnit. MbUnit supports the same row and data fixtures as MSTest, given MSTest supports database driven fixtures but MbUnit has XML driven fixtures and then add's a load more different fixtures. MbUhit shares the same kind of license as NUnit and like NUnit is opensource. I am still only scatching the surface, take a look at some of the past comments to see why folks are into MbUnit.
MbUnit has a wiki, mailing lists and IRC channel so if you need help getting started or want to help out then your very welcome.