The Gallio Automation Platform is an open, extensible, and neutral system for .NET that provides a common object model, runtime services and tools (such as test runners) that may be leveraged by any number of test frameworks.

Main site: http://www.gallio.org/

MbUnit is an extensible unit testing framework for the .NET Framework that takes in and goes beyond xUnit pattern testing.

Main site: http://www.mbunit.com/