JMX looks like a brilliant way to handle application deployment and configuration. Most articles seem to be concentrating on how it could be used for pluggable framework configuration, such as cache management, database access etc. I think it looks like a fantastic way to allow hot-configuration of running applications without resorting to the usual property file hacking. Just wrap your application in an MBean exposing the configuration properties (database URL, resource locations etc), and it becomes trivial to stick a web interface on the front and reconfigure on the fly, or programmatically in response to events. It shouldn’t be too hard to implement clustering as well, so changing the settings of one server in a farm propagates to all of them. JBoss are quite far ahead with this line of thought, it seems – their clustering stuff looks very interesting.
Javagroups is a cool multicast framework (used by JBoss’s clustering mechanism).