"Kwyjibo. A fat, dumb, balding North American ape with no chin and a short temper." - Bart Simpson
Kwyjibo is a little library that allows your users to simulate and test error handling logic. By including a magic string of your choice in one of their input fields, they can thereby trigger an exception (such as a network failure) that would otherwise be difficult and time consuming to test, and ensure that the response of your code to this condition is what they expected it to be.
For usage and examples, see the unit test project.