Risc OS Open have announced the official release of Risc OS Pi. It’s available to download now (follow the link) and is actually hosted on the Foundation website, although they’ve yet to provide a link to it from their downloads page.
Risc OS used to be all the rage in the 80s/90s and is one of those “nothing wrong with it” operating systems that a) never made it to mainstream and b) you should try just if you like it.
Go get!
RISC OS Open: News: RISC OS Pi released (RISC OS for the Raspberry Pi).