Mine is the N800 by Nokia:
The newest is the N810
Ubuntu is going to be able to run on this with the new 9.04 Jaunty Jackalope with support for the ARM processors! ^_^
so far there has been a port that is able to run a debian linux on the little thing, aloooot of functionality for such a teeny device.
Runs a Nokia Linux called Maemo
Nifty on-board apps + a whole community of aspiring development