this is the only java app that I really use in my n900. It works very nicely. Just search for "java is possible in n900" without quotes, get java, those headful tailful things from sun, not openJDK(which seems to consume three times more virtual memory), just follow how to guide of your-freedom for *unix. And yes even open vpn works great, you just have to put sudo in front of command (I have created .desktop file), the other method didn't worked for me.

I would suggest you to get a bit old version, if available, the developer implemented the gui flow/resizing to allow it to resize to "N900" screen resolution as well but it goes blank white after using for some time, developer is also N900 user, nice guy.