Java tools

I have been using cobertura & findbugs for the past 6 months. I have to say that they are some of the best tools that I have used so far.
http://cobertura.sourceforge.net/

FindBugs™ - Find Bugs in Java Programs
The fixit at Google showcased new capabilities of FindBugs that provide a cloud computing / social networking backdrop…findbugs.sourceforge.net

If you do follow TDD make sure that you use these tools.