If you suffer from cold hands during the winter but still want to actually ride your bike outdoors BarMitts are probably the best solution. The picture above is the Mountain/Commuter model.
Here's a bad picture of what your arm looks like inside. With BarMitts you can wear a substantially less bulky glove in colder temperatures without being cold. This translates to increased dexterity. Your hands aren't bound by a large bulky glove and therefore using the controls (brakes, shifters) is much easier.
Here's a picture of my bike on a ride this weekend. The air temp was about 20F with moderate winds. I rode for four hours with just a light liner glove and was perfectly comfortable the entire time. BarMitts are constructed of 5mm neoprene and come in configurations for Shimano, Sram/Campy, and mountain. The road models are sized small, medium, and large which just changes the diameter of the opening by about an inch per size. I very much look forward to using this product for the rest of the winter as I won't have to be on the trainer anywhere near as much!
-CTBF