Anything that has been cross-compiled for the arm processor will run - just install stuff as normal.

I have Easy Debian installed on my n900 and it breathes new life into the device.
I uninstalled openoffice because I don't have the patience for it and I wanted to free-up disk space. But I installed gimp & inkscape and they are perfectly usable.

I've just installed some programming tools too (build-essential).

It's very handy.

Just try it and see.