Chet Haase blogs about how they are looking for Java applications which they want to improve on either the UI or the performance side:
It's Sun, so they probably don't want SWT applications, which leaves off Azureus. However, there must be other apps out there and I for one am going to look around. The deadline is March 31st.
You can also do this yourself, of course. Personally, I'm not very interested in the UI side, but more on the performance side. I'd say if you haven't ever used a profiler, pick an application which you think needs some performance improvement, download a profiler and run it on your application. One profiler I could recommend is JProfiler. It has a 14 day trial, but that's no problem since it passed my 15_minute_test with flying colors. I've also seen people put the built-in profiler of Eclipse put to good use.