A couple of good sources for those who need to decide:
Capability comparison made by Morgan Soley. Half year old but with a feature comparison table.
Recently updated comparison by Andrew Kazyrevich.
TypeMock Isolator is a clear winner if it fits in your budget. Otherwise Rhino Mocks or Moq seem to be the most mature frameworks. According to a recent Roy Osherove’s pole they are in the lead with Rhino Mocks having 47% and Moq on second place with 34%. If I had to choose a free mocking framework today, I’d probably go with Moq because of Rhino Mocks’ confusing “very large array of syntaxes”. Or wait for Rhino Mocks 4.0 that will be radically simplified and, like Isolator, will go for AAA abd single “fake” concept.
But for mocking without severe limitations there is no alternative to Isolator. Still in the lead.