The Pragmatic Craftsman :: Simplicity from complexity : by Stanley Kubasek ::

PMD: A static Java code analyzer that includes a boatload of builtin rules and supports the ability to write custom rules. CPD (the Copy/Paste Detector) is an add-on to PMD that uses a clever set of algorithms to find duplicated code. (http://pmd.sourceforge.net)

Checkstyle: A highly configurable coding standard checker with a default set of standard and optional checks. (http://checkstyle.sourceforge.net)

Cobertura: A code coverage analyzer that identifies areas of code that aren’t covered by tests. Unfortunately, code coverage metrics are often used as instruments for programmer (and tester) abuse. When used properly as constructive feedback, these metrics can help improve your testing skills by identifying aspects of code that often go untested. (http://cobertura.sourceforge.net)

JDepend: A static code analyzer that generates design-quality metrics based on Java package dependencies, including identifying circular package references. Disclaimer: Your humble author wrote this tool. (http://www.clarkware.com/software/JDepend.html)