Ubuntu One's killer app is Tomboy[1] which can use it to synchronize across computers and Androids[2] and is consistently the app I use the most after the terminal and browser. For everything: scribbling, grocery/shopping/todo lists, things to remember, personal wiki articles...
[1] http://projects.gnome.org/tomboy/features.html [2] https://launchpad.net/tomdroid (use the beta provided as an .apk in the sidebar)