Arapahoe Basin has remained independent and unspoilt since 1946, avoiding being snapped up by Vail Resorts (unlike Keystone and Breckenridge). Even so, it is included on the extended lift pass that covers all the local domains. It is a ski area rather than a resort, with limited accommodation, dining, après-ski and non-skiing activities. But it is only 10km from Keystone and is easily accessible.
A-Basin, or The Legend, as it is locally known, is quite a locals' resort as well. Its lifts are old and slow, but the pass is good value. The main reason to visit A-Basin is for the great snow record. This is owing to its altitude (3285-c.4000m), which allows some of the highest skiing in the US.
It is not a big resort, with only 61 runs, while most of the runs are very short. Some, however, are very challenging and steep, particularly the 'East Wall', offering couloirs and moguls on the opposite side of the bowl. For more advanced skiers it is great fun for a day's visit, but you would probably be best based in Keystone or one of the small towns nearby (Frisco and Dillon are particularly suitable).