Looking for good starting point to learn Apache TomCat. What popular free application uses TomCat so I can do an install and configuration?

Hey folks.

I have never worked with Apache Tomcat. I am looking for a good starting point to get my feet wet with it. I am thinking of installing a popular free application that is based on Tomcat but I have no clue which one to use.

If you have any better pointers/suggestions I am all ears :)