Knoppix 6.7.0 and 6.7.1 DVDs will boot on an iMac, at least they do on mine. I have a 21.5" iMac from early 2009. It's the Intel dual core 3GHz version running Snow Leopard.

The iMac hardware detection seems mostly OK with me & certainly the graphics all work OK, even Compiz effects. What doesn't work for me are the wireless keyboard and the Apple wireless magic mouse. I use the Mac wired keyboard and a 'normal' Logitech wireless mouse.