Sonar tries to identify "hot spots" of complexity and maintainability combining the results of various open source tools (including PMD and Findbugs). It integrates well with Maven and CI servers (especially Hudson).
EDIT by extraneon
There is a Sonar site available where a lot of open source projects are analyzed. I think this shows quite good how much rules get applied,and how far the drill down goes. You can of course also disable rules you don't find that interesting.
Here is an explanation of the metrics.
