picoGUI is a space-efficient windowing system developed for PDA's. It is rather limited in its applications, however, and its development seems to have ended.
You can find out about installing picoGUI on the Psion 5MX from Tader's Psion 5mx Linux Pages. You can find some screen shots there as well.
Grab the binaries (and an actual distribution tarball) from: http://thomas.de-ruiter.cx/projects/psion/files/PicoGUI/ There are two subdirs:
/etc /usrcopy both to the ext2 partition. Then type pgserver. (Note: You get a crappy error message if you've changed the color depth to 2bpp with fbset -a 2bpp.)
There is a binary package for the Zaurus available on: http://pgui.sf.net which is for the ARM as well. Maybe /etc/pgserver.conf needs a little tweeking. But that should work as well.