MagicWheels

MagicWheels are wheelchair wheels with a geared hub that provides a 2:1
mechanical advantage and an automatic hill-holder. They are great for
climbing hills, ramps, curb cuts, or door thresholds. Unlike power assist
wheels, MagicWheels 2-gear wheelchair wheels have no batteries and no motors.
You only use the geared technology when you need it, engaging it with a
simple click of the hub. The remainder of the time your wheelchair works
similarly to how it does with conventional wheelchair wheels installed. 

Read more at:
http://www.magicwheels.com/