A while back I got briefly interested in doing sick and unusual things to a Cybiko. Now completely and justly forgotten, these scarily brightly coloured handheld computerlets sold enough that they're really easy to get hold of these days.
This was interesting, because they were based on the rather nice Renesas H8/300 processor, and were nigh trivial to hack.
I eventually got a port of the equally but unjustly forgotten operating system Prex running on it. I never got round to polishing it or uploading it anywhere, as the number of people in the world who were at all interested in it was, basically, me, but here it is anyway. I got it up to the stage of allowing you to enter commands on the keyboard, read the results on the screen via a framebuffer console, and run programs (very, very, very slowly) via a tiny compressed filesystem. It's probably most useful as a software reference for anyone else who wants to run bare-metal code on the Cybiko.