Get evince if you ever want to view a PDF. Much better than the built-in one.
I like mpd and mmpc for music playing, but whatever your preference, you _must_ get some better media player than the built-in one.
If you want an xterm (you will, for any serious fiddling with the device) get the "flying keyboard" one: http://74.86.202.247/forums/showthread.php?t=10348
If you're a commandline junkie, but not a vi-person, get JOE: http://www.opbyte.it/maemo/
If you want to take notes, highlight PDFs, etc., Xournal!
Because writing stuff up on the tablet, exporting (with notebook paper background) as PDF, and printing from the PC produces lovely results, and confuses little sisters into thinking you borrowed their sharpie collection.