My 2 N900s are still my only phones. Last year I developed a Python GTK habit tracking app on one of them, which I've been using faithfully since to motivate me to do stuff each day/week.

I had set my laptop up using an inotify script to synch changes to my N900 and launch the app each time I wrote one of the source files from vim. That worked pretty well for testing purposes.

If I had time I would try to do more quick and dirty Python/GTK apps.