You know what would be super fun? A distributed MP3 jukebox, using Jini.
Any machine on a LAN could run the Library service and/or Player service. Library would simply offer up a catalog of available MP3 files on that machine, and allow those files to be streamed to clients. Player would allow files to be queued up and played. All of this would be controlled by Swing clients from anywhere on the network.
With such a system, you could use your 802.11b-enabled laptop to have your HTPC start playing a song that is stored on a server in your basement. You'd never have to sync up your MP3 collection across multiple machines. And since it would all be based on Jini, it could all be very robust and decentralized, with no "central server" necessary. New Players and Libraries connecting to the network would be autodiscovered by everyone else.
All of this would be extremely easy to do; the Java Media Framework makes it easy enough to play MP3s, and the rest is just pushing bytes and messages around.
I'd love to build this, if only to give Jini a try. Oh well, tack it on to the end of the "To Do (Maybe)" list...