I've compiled a list of the most useful MBeans to be found in JBoss through its JMX console: Aleksander Adamowski's JBoss JMX console tips and tricks [1].
It includes MBeans with operations for shutting down JBoss remotely, listing Java system properties, listing JNDI contents, forcing log4j configuration reload, listing ClassLoader info for given class, listing deployed apps, retrieving cluster information etc.
This list is being updated whenever I discover some MBean worth mentioning.
Links:
[1] http://docs.google.com/View?id=df5k88n3_155hc3w4pdn