I am currently working on developing a mutation testing tool and I need to find an Open Source Java project which has a relatively good amount of JUnit Tests. The idea is that I'll be using the opensource system and its TestSuite so as to perform a case study on my mutation testing tool. Thanks for your help :).
I too have written a mutation testing tool.
I recommend that you make sure your mutation tester detects the unnecessary code in Bob Martin's Bowling Scorer game as described here. It's not much code so it's a quick check.