Relatively cheap, this is the first time I've seen them. I ordered one. Maybe they'll plummet in price as the stepper motors and ultrasonic sensors did over the next few months.
Looks to be a ball bearing unit. Might be useful on a robot as a non-powered idler wheel to measure distance for dead reckoning, I reckon.
"When you make a thing, a thing that is new, it is so complicated making it that it is bound to be ugly. But those that make it after you, they don’t have to worry about making it. And they can make it pretty, and so everybody can like it when others make it after you."
- Pablo Picasso