Heckle is a mutation tester.  It modifies your code and runs your tests
to make sure they fail.  The idea is that if code can be changed and
your tests don't notice, either that code isn't being covered or it
doesn't do anything.

It's like hiring a white-hat hacker to try to break into your server
and making sure you detect it.  You learn the most by trying to break
things and watching the outcome in an act of unit test sadism.

Homepage:
http://seattlerb.rubyforge.org/heckle/
