Glenwood Springs is also home of Sunlight Mtn which has the Sunlight snowmobiling parking lot and trails into the White River National Forest. The Sunlight Mtn Resort offers snowmobile tours.
Glenwood Springs, New Castle, and Rifle all enjoy the Flattops Wilderness, to the north. The Flattoppers is a club that rallies this area.
The Colorado Vacation Directory has a cute map of the flattops area north of Rifle
Vega Reservoir area of the Grand Mesa has snowmobiling trails and cabins. Find Vega and Grand Mesa snowmobiling maps from Colorado Parks and Wildlife here.
General trails for Vega and Grand Mesa can be found here and of course are great for snowshoeing and cross country skiing when snow pack is happening.
Grand Mesa trail maps for cross country skiing and snowshoeing including trail maps for Powderhorn Ski Area can be found here.
The Grand Mesa Nordic Council has a lot of information on the trails they maintain and skiing activities in Mesa county.
The Aspen Snowmass Nordic Trail System website has all the information for trails from Aspen to Basalt.
Spring Gulch Cross Country Trail System is a great option in the Carbondale area.
The Colorado Snowmobiling Association has a great club finder features.