The Mantis Shrimp (http://en.wikipedia.org/wiki/Mantis_shrimp) forms a crude wheel to maneuver on land.
But I still can't think of any examples of wheels in nature that use axles and are large enough to be more than a free floating rotating object. Maybe this seems an arbitrary threshold, but I think usually when we marvel at the wheel, we're marvelling at axles, and their ability to support weight and radically reduce friction when moving big heavy things, all while holding the object basically level. While the cellular turbines that power us are pretty fascinating in their own right, it'd be interesting if anyone could think of biological wheels with axles that were a bit bigger. So far, I can't think of any outside of science fiction.*
*Pullman's "The Amber Spyglass" even gives a somewhat plausible evolutionary background for his axled and wheeled Mulefa. Any others?