PMD

http://pmd.sourceforge.net/
PMD scans Java source code and looks for potential problems like:

Empty try/catch/finally/switch blocks

Unused local variables, parameters and private methods

Empty if/while statements

Overcomplicated expressions - unnecessary if statements, for loops that could be while loops

Classes with high Cyclomatic Complexity measurements
 http://www-900.ibm.com/developerworks/cn/java/j-pmd/index.shtml
 http://www.onjava.com/pub/a/onjava/2003/02/12/static_analysis.html