I remember a day last winter when I arrived at my depot and the dispatch due to -40 temps were handing out chem packs that fit into socks and one fellow even had a pair of socks designed to accept them in lil pockets. the packs were thin and would fit into winter boots n such, and worked for 4-5 hours before you needed a new one.
I already had a sweet foot arangement so I didnt bother with them but I took my share anyway and put them in my hip pouch with my nextel/pager and phone which can act up at those temps + windchill. They kept my gear working and even better kept my pens from freezing.
The guys that wore them love them though.