Given that I'm speaking on Python mocking shortly, I thought I'd better dig through my accumulated bookmarks and see what I'm missing.
PyMock is what I've been using up till now. It's heavily inspired by EasyMock, a mocking style I'm used to. It's pretty good, really. The docs really could be better, and the failure messages are not very helpful, but it does the job.
But then I found Mox. Superficially, it's very similar to PyMock. It's also based on EasyMock. But wherever Mox does differ from PyMock, it's better.
It's much better documented than PyDoc, and has more meaningful, helpful failure messages. I think it's has more functionality, too, with some nice comparators, side effects and callbacks. StubOutWithMock() is nice too. But it's possible that PyMock has all of this - given the documentation, it's hard to tell.
Mox will warn you if you call a method on a mock that the mocked class doesn't have, which is handy. It can get confused if your mocked class uses delegation, but you can always fall back to MockAnything(). The mailing list is small but helpful.
On the downside, Mox isn't in PyPI, which is a shame. And they missed a trick not calling the documentation MoxDox. ;-)
There are many other Python mocking packages out there, too - notably pMock, a jMock style mocking library, Mock and minimock, a mocking library for the doctest unit test library.
In early development there's Mockito For Python, a port of Mockito, which will be worth keeping an eye on if Szczepan ever gets the hang of Python. ;-)