This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
Personally I use the Tomcat Manager App for this purpose. It is part of the tomcat 6 standard installation but you have to configure a user and password to get to it. It will show you all current request Threads, memory use, etc.
The other day, I found a free alternative to the Tomcat Manager: http://www.lambdaprobe.org Perhaps not very active in development, as of lately, but it works well with Tomcat 6 when I checked.