OCMock is an Objective-C implementation of mock objects. If you are unfamiliar with the concept of mock objects please visit mockobjects.com which has more details and discussions about this approach to testing software.
This implementation fully utilises the dynamic nature of Objective-C. It creates mock objects on the fly and uses the trampoline pattern so that you can define expectations and stubs using the same syntax that you use to call methods. No strings, no @selector, just method invocations like this:
[[myMockObject expect] doSomethingWithObject:someObject];
How to have fun with OCMock