QALab consolidates data from Checkstyle, PMD, FindBugs and Simian and displays it in one consolidated view.
QALab keeps a track of the changes over time, thereby allowing you to see trends over time. You can tell weather the number of violations has increased or decreased - on a per file basis, or for the entire project. It also plots charts of this data.
QALab plugs in to maven or ant.