I've been thinking for a while that it'd be great if there were a Linux-like project to create a true Java operating system, and what do you know? here it is...
Lately, I've been working on a package called Q2Java, which lets you write Quake II games in Java. It's not directly JOS related, but in doing it I've learned a -lot- about Java, JNI, Reflection, and implementing a simple game plug-in system.
Hopefully, some of this will be useful in JOS, and I'm sure that at least the JOS javax.vecmath package will be useful to Q2Java.
Another Java game-related package I've put out is a "Total Annihilation" HPI-file extractor (something like an Unzip program)
Otherwise, most of my other programming experience has been with things such as e-mail servers, web-servers, Java servlets, and various command-line utility-type programs.