Visualvm is able to measure application performance (profiler) and monitor system resources.
It’s really a nice tool if you need to monitor your tomcat and get some informations about threads, memory usage, cpu load, loaded classes, etc.

http://visualvm.java.net/

I’d say this application is fundamental to watch over your tomcat …

• Screencast: http://blip.tv/kg/introduction-to-java-visualvm-1591181
• Eclipse launcher: http://visualvm.java.net/eclipse-launcher.html