The Futon mouse pad plugs into a computer through USB, and it will certainly keep your hand plenty warm. In fact, it can reach temperatures up to 50ºC (122ºF). That might do more than keep your hands warm, so if you actually get your hands on one of these be careful not to turn it up all the way, or you might just burn yourself.
The mouse pad even comes with a comfy little wrist pillow to give you support. If it's not exactly cold enough for you to use the heater, but you still want a blanket for your mouse hand, you can pull the heater out and still use the cover to keep your hand protected from the harsh elements inside your home or office.
This is definitely the kind of product that could only come from Thanko. The company regularly releases some of the craziest products, and this mouse pad is clearly no exception. Still, it actually has some practical use, especially for people with poor circulation. It is available for ¥1,980, or just under US$25.