PMD scans Java source code and looks for potential problems like:
You can download everything from here, and you can get an overview of all the rules at the rulesets index page.
PMD is integrated with JDeveloper, Eclipse, JEdit, JBuilder, BlueJ, CodeGuide, NetBeans/Sun Java Studio Enterprise/Creator, IntelliJ IDEA, TextPad, Maven, Ant, Gel, JCreator, and Emacs.
2011-11-04 PMD 4.3 (download):
The refactoring effort that will lead to PMD 5.0 has been started on the main trunk (5.0 will not be backward compatible with PMD 4.x). We may release other 4.2.x versions with minor fixes if needed until 5.0 is released.
A snapshot of the web site for the new version is generated daily by our continous integration server. Early access versions can also be downloaded from there.