RISC OS computers - the name by which what were once called Acorn computers are now known - made a name for themselves principally in schools computing in the UK and a few other countries. They use a proprietary operating system called (unsurprisingly) RISC OS. The desktop version of the OS is managed by RISCOS Ltd, and corporate matters are managed by Castle Technology. A new (and cool) variety of RISC OS machine is the A9Home, available from Advantage Six Ltd. It's a small box (about 6.6 inches by 4.1 inches by 2.1 inches) in a nice shade of metallic blue - and it draws only a bare 5 watts or so, which means it hardly gets warm at all.
The Acorn community - for that is how it feels to those who use Acorn-badged machines and their successors - is a friendly place. It tends to be very helpful and supportive to newcomers and seekers after RISC OS truth. On USENET, the comp.sys.acorn.* newsgroups are a treasury of information and news.
For a way of running an Acorn machine on your PC (or, in a beta version, on a Macintosh), head over to Virtual Acorn.
For RISC OS resources, take a look at My RISC OS, acornusers.org, RISCOS.org, Qercus magazine, Archive magazine and The RISC OS Products Directory.
For RISC OS hardware, try Castle (who make the IYONIX pc) or Advantage6 (makers of the cute blue A9home computer) as a first approximation.
For help on RISC OS, visit Richard Atterer's excellent site or see the brief introduction to Acorn and RISC OS by Richard Butler. The FAQs for the comp.sys.acorn newsgroups are a valuable resource too.