Perhaps the most wonderful thing about cross country skiing is the nearly limitless opportunities the sport provides. Unlike XC's much-hyped, great-great-grandchildren (aka. alpine and snowboarding), XC skiing does not require mountain ranges, mechanical manipulation, oversize clothing or a fully-stocked apres-ski lounge in order to enjoy the activity. As long as you have snow (and on roller skis or sand skiing even that isn't needed) you can go out and celebrate the outdoors!
Thus, even the most comprehensive listing of actual places to XC skiing is a very partial list. Rather than embark on a futile effort to describe every resort, forest, mountain, golfcourse, etc. that one might find excellent XC skiing around the world, what is offered here is a general description of the opportunities on each continent along with valuable features on specific spots to ski around the world.
The only out-of-bounds is your imagination.