Pro Spring development mostly choose Tomcat as the web container. Tomcat is lighter and more agile if developers try to avoid huge, heavier, full blow version JEE server. However, one thing missing in Tomcat, and I really miss a lot in big O's OC4J and Weblogic is the management and monitor tools. 

Admin interface in Tomcat is shambolic and useless. You have to tail and trace the log to understand exact problem where is. So more hardcore developers go for the low maintenance, less configuration, easily clustered Jetty instead. Anyway, this is another debate or a new war Tomcat vs. Jetty.

Now, turn to Java Melody, an easily setup and integrated with your existing Java web application tool, let you monitor application's performance, configuration, JVM inside, SQL connection, cache, application error and warning ...

Java Melody is the tool web application development must have.

Read more on pictures: http://code.google.com/p/javamelody/wiki/Screenshots