RISC OS for Raspberry Pi. Eben writes -

We’re very pleased to announce the immediate availability of RISC OS for the Raspberry Pi. First released in 1987, its origins can be traced back to the original team that developed the ARM microprocessor. RISC OS is owned by Castle Technology Ltd, and maintained by RISC OS Open Ltd. This version is made available free of charge to Raspberry Pi users. We really recommend a download; it’s very smooth, very fast to boot, and we’re delighted to be able to offer RISC OS for the platform. You’ll find a RISC OS image on the downloads page; and if you’re completely new to the OS, you’ll find Burngate’s  Stroll around RISC OS in pdf form very useful.