Listening to #115 of the Java Posse in traffic this morning (taking the car to the mechanics), the boys have mentioned it many times before, the findBugs plugin.
I finally remembered again to go back to the podcast listing http://javaposse.com and take a look.
FindBugs looks great, I quickly ran it across two projects I had open in Eclipse and it certainly picked up a 3 in one and about 28 in the other.
Going by class (I should go by LOC) the 28 bugs on 60 classes is a bad.
The other is 20 classes and 3 bugs .. good thing this second project is recent :-) and the other 3 years active.
Good job for that plugin I say.