Maybe it's not the simplest, but apart from running on top of a sequoia log, I can't think of any simpler unicycle than this. $74 will get you the Nimbus Ultimate Wheel with no seat and no chain. However, it comes with lots of free stuff, plus a demo video after the jump.

Free stuff like one broken arm, two broken femurs, one tibia, one or both hips, some internal bleeding here and there, probably some brain damage, chronic vertigo from inner ear lesions and a big increase in your health insurance premium. Unless you are one of the people racing on them in this video.

[Nimbus Ultimate Wheel via Geekologie]