RISC OS used to have two options available when it came to hardware: Castle's Iyonix and Advantage6' A9Home. Sadly, the former is out of production, and we haven't heard a thing from the latter in ages. Then again, these machines weren't screaming when they first came out anyway - "Things that are taken for granted on a Windows or Mac OS machine, such as watching video, are simply impossible on current generation RISC OS hardware."
There is another way to run RISC OS: emulation. You can use the free and open source RPCEmu, or the commercial, for-pay VirtualAcorn. Still, this is just emulation - real men don't virtualise. They run native.
Luckily, though, the Castle branch of RISC OS (more on the RISC OS fragmentation) has had its code opened up, so one could start dreaming about maybe porting RISC OS to more modern (more available) hardware. This is actually what's going on today (we've reported on it before): RISC OS is being ported to the OMAP line of ARM processors, with the test bed being the Beagle Board.
The Beagle Board is a tiny SoC built around a Texas Instruments OMAP3530 processor, which runs at up to 600Mhz with 256kb of L2 cache. It also has a TMS320C64x+ HD video capable DSP, so it's actually pretty powerful. Port-wise, it has DVI-D, an MMC+/SD/SDIO slot, s-video, and audio ports, and you can power it over USB. It has limitations too: 256MB of RAM is the upper limit, and it can only do 1280x1024.
RISC OS is currently being ported to this board, which is available for only 149 USD. However, the long periods of stagnation in the development history of RISC OS make the porting effort a daunting one indeed. Still, a lot of progress has been made, and the Beagle Board can now more or less run RISC OS (see the screenshot below), but limitations remain: no video acceleration, no audio, and there are issues with the CMOS settings.
Getting RISC OS to run on the Beagle Board requires some old fashioned handywork which you virtualisation-spoilt brats of today might not be familiar with, but then again, if you own a Beagle Board, you are most likely willing to get your hands dirty anyway. The RISC OS 5 OMAP build can be downloaded from the RISC OS Open website.
If you have a Beagle Board, you can help the effort too by testing it out and reporting bugs. If you have the skills, you can help the coding and porting effort in more direct ways, too, of course.