Findbugs is one of my favorite static analysis tools. It's goal in life is to enables you to isolate and correct dangerous Java bugs in your code, and it does this particularly well. I find that the issues raise by FindBugs are very often worthy of attention - indeed, the issues it finds can be particularly knarly ones, and the "false-positive" rate is suprisingly low for this type of tool.

If you want to find out a little more about FindBugs, and especially how it compares to other static analysis tools on the market, check out this article on DevX.