XWiki SAS is part of a research project named STAMP. As part of this project I've been able to experiment a bit with Descartes, a mutation engine for PIT.
What PIT does is mutate the code under test and check if the existing test suite is able to detect those mutations. In other words, it checks the quality of your test suite.
Descartes plugs into PIT by providing a set of specific mutators. For example one mutator will replace the output of methods by some fixed value (for example a method returning a boolean will always return true). Another will remove the content of void methods. It then generates a report.
Here's an example on a module of XWiki:
