I installed #CheckStyle this week. I give it 5 stars for usability, customisability and features.
Checkstyle is a development tool to help programmers write #Java code that adheres to a coding standard. It automates the process of checking Java code to spare humans of this boring (but important) task. This makes it ideal for projects that want to enforce a coding standard.
Checkstyle is highly configurable and can be made to support almost any coding standard. An example configuration file is supplied supporting the Sun Code Conventions. As well, other sample configuration files are supplied for other well known conventions.
A good example of a report that can be produced using Checkstyle and Maven can be seen here.