Ski-bikes have been around for generations but most are for downhill skiing.
The SnoMoBike can be pedalled so it's like a cross-country MTB.
Other solutions to snow riding include Surly's Pugsley, a bike that can take 4inch tyres on 26inch wheels, and the recumbent hand-trike SnowPod.
The SnoMoBike has a ski instead of a front wheel and a tank-track type tread unit on the rear.
Thanks to: http://www.velovision.co.uk for SnowPod link.