I was very excited when Squirrel suggested a topic about mutation testing. I had looked at Jester & Jumble before, but gave up after a couple of hours, as they are not very easy to setup, and mostly not maintained.
The idea of those tools is to make changes to the source code (Jester) or the bytecode (Jumble), and check if the tests still pass. If they are well written, they should fail when the code is changed. If not, then the code is not covered properly. Ideally, you should only have one test failing; otherwise, it means that you have redundant tests.
Ivan, the creator of Jester, was present, but admitted that he had not even used it in years. After seeing the interest of the participants, he seems willing to give it more time. Hurray!
Check out my notes on the conference wiki.