We're doing Coach Fred's (RoadBikeRider) Power to the Pedals 12 week routine on spinner bikes at the health club this winter- twice a week one hour sessions; did it last winter and results were pretty amazing. ALso do one of the aerobics spinning sessions on Sat AM, and ride outdoors on Sat or Sun when weather permits (just rode outdoors today for the first time in Jan).
edit: not as an organized class, just my wife and I, and a riding buddy.