I am in the process of making hand warmers for people at work. Its flannel fabric with short grain brown rice in it. I also added a little wristlet made of some unused shoelaces so that they don't have to put them down, just let them go. You put them in the microwave for 30 seconds and they stay warm for quite a while. This pair has gone to my grandma who kept exclaiming how they were still warm so long after being out of the microwave and tossed around the table.
Prior to this I had made one out of a bandana so it can go around my neck/shoulders, filled with lots of rice. I microwave it for longer, but while sitting next to it on my desk I was struck with the idea for the hand ones. Its cheaper or the same price as those single use ones and at work we have a microwave in the cafe by the registers so people can just go over and rewarm them. They are also reasonably small enough so they could fit inside mittens or even on the palms of gloves. Its not usually my hand that gets cold in gloves, but my fingers. So that may not be as awesome, but that's also why I don't like gloves, they make my fingers colder not warmer.