In no particular order:
1. Portabase (I use it as a media database as well as a ding based dictionary)
2. Aard Dictionary (A very good offline wikipedia.The only thing that disturbes a little, is that it often asks to make an internet connection without apparent reason...but that probably can be tweaked)
3. Scout - no other way to search in Greek contacts
4. Ukeyboard
5. Calendar - home widget
6. Easylist - as a shopping list
7. maep (with preloaded Openstreetmaps from my Desktop Marble Globe)
8. GPS Recorder
9. FileBox (because the standard file manager is nearly unusable)
10. MidnightCommander (because sometimes you need something more powerful than FileBox)
11. leafpad (because I never learned to use vi)
12. Catorize (because I have installed way to many applications)
13. Brain party
14. Evince (because it supports bookmarks)
15. AlmostTI (because it is great)...unfortunatelly the SpeedcrunchPort for maemo has a strange keyboard layout.