Open Source / Free alternative of Typemock Isolator

Disclaimer: I work at Typemock

Typemock Isolator utilizes the CLR Profiler API to perform IL rewriting, thus allowing you to fake (mock) static methods and concrete classes. As far as I know, Moles does similar things to Isolator in terms of using the Profiler to mock those things, and it's free (but not open source).

All other open source frameworks using the Dynamic Proxy to implement interfaces and abstract classes in runtime, therefore they cannot mock sealed/static classes.

I'm not familiar with all of the features of Typemock Isolator, but Moles is a free product that I've used for unit testing.