OK, so it doesn't support texturing, lighting or any other kind of shading, doesn't use a standard API, doesn't support hardware acceleration, and probably needs some serious optimisation in order to cope with complex scenes ...

... but it is free, open-source, fast enough to make some decent games, and most importantly it runs on RISC OS.

If you're interested in helping out (graphics wise or games programming wise) head over to Coder's Cauldron and take a look