Kebler Pass Road is paved for the first little bit out of Crested Butte, but eventually it turns to gravel and runs for many miles as a gravel road over the pass. This road runs through one of the largest aspen groves in the world, making it a great ride--even if it isn't singletrack.
Kebler Pass
http://www.singletracks.com/item.php?c=1&i=9243