The Risc OS is a small and fast operating system designed for ARM processors, in fact Acorn who originally designed the RiscOS also developed the ARM processor and created the BBC Micro computer used by schools in the UK to teach computing. So it seems poetic to run the RiscOS on an ARM powered Raspberry Pi designed to teach programming in schools.
The RiscOS Open is the version that has been made available for the Raspberry Pi, maintained mainly by volunteers and the community. There is a wide range of applications available both free and commercial, for everything your would expect such as productivity, graphics, programming, entertainment and surfing the net.
This RiscOS is most suited to desktop applications rather than networking and the Internet so has a good file management system, uses 3 button mouse controls and has great drag and drop functionality. You can drag files from one application to another via the save option automatically loading the file into the other application or drag it into a directory to save it, no need to browse through umpteen folders in a file requester to save your file. There is an icon bar at the base of the screen which is used as the central control for the desktop. Unlike other operating systems where the icon bar is a combination of your favorite programs and other programs that you have opened, the RiscOS Icon Bar is used to access running programs, load files to them and access the file system using the drag and drop features including connected USB drives and devices and memory cards.
If you do remember programming a BBC Micro computer then your be pleased to hear that BBC Basic is part of the RiscOS and only a press of F12 away. So you can dust off that part of you memory and see what you can still remember.
There is also a good introduction to the RiscOS at www.svrsig.org/Beta.htm
A list of compatible software is available at http://www.riscosopen.org/wiki/documentation/show/ARMv6%2Fv7%20software%20compatibility%20list
The RiscOS is worth considering if you are looking into what operating systems to run on your Raspberry Pi.