Ebook reader - i read a lot of books on my phone FBreader n900 & debian version FTW
torrent downloader - i'm not using n900 torrent clients, but debian transmission client(i find it superb compared to version in extras)
ssh - i'm connecting to my router to reset him or change other stuff
VNC - x11vnc is installed on my n900 so i can remote control it from work
psx4all/DrNokSnes + sixsaxis + tv-out - i love my portable psx/snes console
mplayer + midnight commander - sometimes I like to watch my fav tv shows like Dr House or That 70s Show. Mc is for picking the episodes
Open Office from debian - it's just needed in some situations
Kadu - to webchat with friends
MicroB + Opera Mobile - I use the microB browser when i'm connected to wifi and OM when i change connection to 3g

I think it covers main usage of my n900