Heated grips

Dual Star and Kimpex make grip-heater kits that install under your existing grips.

Hot Grips are also available, but they require you to epoxy their own grips to your bars. Their grips are hard and plasticky. I had Hot Grips and hated them. They failed after a winter due to the wire breaking inside, and since it's all sealed up you can't really resolder any connections...

Eric's guide to wiring up Dual Star grips with a Heat-Troller is here.