The reason I bought the Nokia N800 is because it's the only programmable open source portable device I know of.
As a writer, the most important use for me is, well, writing.
I use emacs v. 22 in a Xterminal with all my usual keybindings.
I pair it with a bluetooth keyboard and I write in cafés and all kinds of places.
I installed scratchbox and the maemo SDK for educational purposes (maemo is debian based, like ubuntu on my desktop), and also because I want to cross-compile aspell (since I haven't found a package ready for download).
The Nokia N800 replaced a Palm T3 which I used for the same purposes (with many more grievances).