Nordic Skiing
Nordic or cross-country skiing is probably the nearest equivalent to running on skis. Unlike Alpine or downhill skiing you can easily propel yourself forward on the flat and uphill. I guess it is akin to cycling in that you do the work on the flat and uphill but gravity helps on the downhills. It's a sport that any runner can do given a little tuition and the right conditions.
This section gives a very brief introduction to getting started with one part of Nordic skiing, namely track skiing. It's far from comprehensive but should point you in the right direction if you'd like to give it a go. It is worth it too as it is great fun as well as great exercise, very good cross-training!
Also included here are the related activities of rollerskiing and biathlon. Rollerskiing is a useful summer training activity for XC skiers but it is now very much a sport in its own right. Biathlon falls under a different governing body to Nordic skiing but it is essentially a mix of XC skiing and rifle shooting.
