Well, the last time I had the opportunity to buy a very cheap iMac with PowerPC (RISC) processor, a friend of mine told the seller what we were gonna use it for; highspeed photography of it. While it explodes from being dropped more than 10 meters... (we're both photographers, and I have sound triggers for the flashes, which means we can get consistently solid results). Upon hearing that, the seller refused to sell us the iMac...
Such old technology is of no practical use nowadays; it consumes way too much power - and even your mobile phone can probably match the processing power. At a fraction of the power consumption. If you're good with tools, you can maybe use it to rebuild it into a cool speaker or a stand for a table top, but that's about as much use as I imagine one can find for it.
PowerPCs were the competitors to the Pentium II - 4 generations, if my memory serves me correct, and they were pretty much on par (well, AMD won that race back then, with their Athlon 64 / 64 X2) - and I couldn't for the life of me find anything I'd like to use such systems for. It's like trying to find a practical use for an old pair of worn trousers that're too small. Sure, you could try and make them into shorts which wouldn't solve the size issue at the waist though - but the funniest thing would probably be to make them into a bag or something completely different... ^_^