Easy debian is fully usable. It is very easy to install. For me the best part of Easy Debian is that it brings a full powered word processor and spreasheet application to my device in the form of OpenOffice.org. However, I have to agree the MicroB is the king of all mobile applications anywhere.

The thing is, the N900 doesn't just have one killer app. It has many, on several different categories. The media player has some great features, but it is probably the weakest aspect of the device. However it does the job acceptly well for me.

Any way, these are my personal killer apps:

1. MicroB
2. EasyDebian/Oo.o
3. Garnet VM (Palm OS on the N900
4. Python