RISC OS is a computer operating system designed in Cambridge, England by Acorn. 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, and maintained by RISC OS Open. This version is made available free of charge to Raspberry Pi users.

Moderator's Note: RISC OS is under a Shared-Source licence that places restrictions on for-profit sub-licensing. Otherwise it is free to use, modify and redistribute.