The main idea behind SkiMap is to allow users to upload maps from various ski resorts and share them with other users. Recently, however, they added a feature that uses the OpenStreetMap API to convert OSM data into KML vector ski maps. In just the past month, 940 ski maps were generated for use on the site!
The quality of the maps varies by location — European resorts are more fully mapped than US ones, and in the US they’re in better shape on the eastern half of the country. However, with the rate at which new ones are being added, your favorite ski area will probably look good on there very soon.
To see a good example of a ski area, check out Whistler Blackcomb. They have 67 different printed maps for Whistler, along with a handful of “OpenSkiMaps”. Here’s a KML file from one of them if you want to see it in Google Earth.
What site do you like to use before you hit the slopes?