Ktrak's traction boosting Rear Drive Kit ($400), which consists of a composite wheel with a polyurethane track, is the first step in turning your mountain bike into a muscle powered snowmobile. To complete the transformation, the Ski Kit ($130) swaps the front wheel with a single, steerable ski. The kits work with nearly any mountain bike.

The rear drive track kit comes in 20- to 26 in. wheel sizes, takes about as 
long to install as changing a tire, and adds grip on sand and snow. 
The front ski (top right) is optional, but fun.