I added a report for FindBugs in our Maven build. It seems to find quite a few issues not detected by Checkstyle, PMD and Simian.

FindBugs is much more targeted towards finding potensial bugs in your code. They use static analysis to inspect the Java bytecode. The list of things being checked is quite long, and growing:

http://findbugs.sourceforge.net/bugDescriptions.html

I first started looking at it after an excellent presentation (TS-1847) by William Pugh and David Hovemeyer at JavaOne this year.