1)First thing I wanted was a good solid case that I could open easily and would protect the screen. Still experimenting with ipaq, ds lite, and some caselogic cases, Ill post an update on the ultimate under $10 :P

1b)After that I add extra repositories for that extra software and hooya!
maemo.org and http://gronmayer.com/n800/repos/ are a good start.
Activate RedPill mode if you're feeling dangerously adventurous.

2)FbReader - thanks to the mighty internets, I either go to http://manybooks.net/
or gutenberg for free books. And then you can find lots of books in english in Russian and Polish sites if you use their proxies :P
There are lots of torrents too with sci-fi and other books. I have about 100 books on my n800 right now.

3)Entertainment - I download movies and watch them in the default player or mplayer. THis is a big topic in itself if you're not familiar with formats and all that.
can be challenging

4)System apps for them console jockey needs!
xterm - a must!
ssh and sshd - a must!
vnc viewer
ping, tracerounte, wlan status, etc. Everything I need. If you need these, you will know :P
5)entertainemnt- lxdoom and battle for wesnoth. Nethack for those good old days.
6)now working on the os 2008 upgrade

Oh how I wish someone would port fluxbox to the nokia n800!