If you want to run Java apps (or J2ME apps), you want to check out the package icedtea6 and microemulator. Do a forum search and you'll find a couple useful threads about both of these. Both have versions in extras-testing, and can be easily installed from there.

Icedtea6 is an optimized Java SE setup for arm processors (like our N900 has) and gives you the ability to run most Java based programs. Microemulator (which requires JSE or icedtea) is a class emulator that can run most J2ME packages within the JSE environment.