PMD

PMD is a useful static source code analyzer. As a developer for a similar tool for Perl, I'm quite familiar with the challenges of such a project.

At my organization, PMD is largely eclipsed by the closed-source tool, CodePro, which has a much larger set of policies and better IDE integration. But the two projects have some disjoint policy sets, so I find it useful to use both PMD and CodePro (plus FindBugs) on projects.