IGN is the French national mapping agency that puts out topographical maps for the entire country. I don't carry any IGN maps at the moment, but hope to someday.

Utagawa VTT offers hundreds of MTB trails in France, Europe and other places for free. Most of the tracks are in the French Alps for the moment. The trails are described with GPS track, profile, maps, pictures. There is also a forum allowing members to discuss about new spots. Members will soon be able to post their own tracks.

The site is in French, but using Google translator allows non-French speakers to convert it in to their language.

A write up on a multi-day guided tour of Les Arcs with Trail Addiction
Video from Les Arcs: