From applications I use on daily basis with my N800:

- book reader (FBreader)
- in fact anything which includes longer texts it is often easier to read in portrait mode including: www browser, PDF reader, (any office docs reader), RSS browser, even terminal sometimes would be handy in that way
- PIM programs which often - at least for me - includes longer lists; easier to grasp things when everything is on one screen (Agendus in portrait mode with hidden Graffiti area rocks!)
- maps: orientate map that your route is along longer axis and you don't have to "refocus" for longer time