Question: What system do you recommend for implementing "automatic updates" for some java (desktop) application? I'd also want to have module/plugin installation and update. Think Eclipse and Firefox.

I'm aware of Eclipse RCP's OSGi stuff, and also NetBeans RCP has something here. Do one have to buy wholly into their respective RCP stuff to use their module and automatic update systems?

Are there other options?

Answer: 
Web Start is great if you have infrequent updates and good bandwidth.

If not, however, you need something else. I worked on a project where we had frequent updates and horrible bandwidth. We rolled our own classfile server; the startup code would talk to the server to see if updates were needed on a per-class basis. If so, the changed classes were downloaded and the application continued as normal.

It was actually not terribly complicated, so don't be afraid to roll your own if existing tools don't work.
Take a look at [Equinox p2](http://wiki.eclipse.org/Equinox_p2), which is probably what you meant by "Eclipse RCP's OSGi stuff"... AFAIK you *will* need to OSGi-ify your app to use p2 (which at the very least means making your whole application one big bundle).