Someone in the Ruby community thought it was a good idea to write a piece of software that tested your unit tests.
Heckle runs your standard Ruby unit tests and reports as per normal. Heckle then alters a certain amount of your Ruby source code and then re-executes your unit tests, reporting on the subsequent run against the altered source code. If your unit tests don’t fail after Heckle has mutated your initial source code, then it proves that certain sections of your application either aren’t covered by tests at all or that they aren’t thorough enough.
I don’t know how many people would use a unit testing utility like Heckle, however anything that can be used to increase the robustness of your code is a good thing I think.