Gallio looks like it's going to be awesome once it gets more stable (currently alpha).
It's not just a test framework, but a test automation platform, so it will work with many existing test frameworks (MbUnit, NUnit, xUnit.net) yet be fully extensible, with a number of built-in additional features such as report generation in many formats and code analysis tools.
I've also heard that it will be able to
- output image streams, so for example WatiN test failures can be output as screenshots, so you can see what state the browser was in when the test failed.
- filter by namespace, so you can easily uncheck tests for an entire namespace before running them
Edit: It is indeed out of alpha now. We've tried it at our company and we really hated it. It was horrible to use and very slow. What a shame.