I've only used one, but I've had good results with Michael Foord's Mock: http://www.voidspace.org.uk/python/mock/.
Michael's introduction says it better than I could:
There are already several Python mocking libraries available, so why another one?
Most mocking libraries follow the 'record -> replay' pattern of mocking. I prefer the 'action -> assertion' pattern, which is more readable and intuitive particularly when working with the Python unittest module.
...
It also provides utility functions / objects to assist with testing, particularly monkey patching.
