The Java Runtime Analysis Toolkit is a low overhead, easy to use, open source performance profiler for the Java platform. JRat monitors an application's execution and persists performance measurements. This data can then be viewed and analyzed using the JRat Desktop, a Swing application.
Read the The Quick Start Guide . It contains step-by-step instructions on how to use JRat for the first time. It's easy.
Lots. This project is again being actively developed. Read some of the highlights here .
The latest official release on SourceForge.net is quite old but stable. The current development release has all the latest features and is more representative of the documentation on this site.