FindBugs in Anger
If not, then let me say that FindBugs is a must have tool in the arsenal of any Java developer and any development team that's not using it as part of their regular development practices is incompetent.
Bill Pugh has done a fantastic job making FindBugs a great F/OSS tool which helps detect a large variety of all too common programming mistakes in Java.
If you aren't yet convinced that FindBugs is really useful, let me point out that I've used FindBugs as an expert witness in cases where outsourcing projects had gone wrong and people were arguing about the quality of the delivered code (among other things). You have been warned. :-)