This is from another site:
We (the organization I do feral cat stuff with) use those air-
activated handwarmer thingees (the kind that you buy at sporting
goods stores, you open the package up and once exposed to air, or
sometimes shaken, they heat up for x number of hours) -- but we use
them for keeping food unfrozen long enough for kits to eat.
Generally we put one under the food dish (say on a styrofoam plate,
then another plate on top with the food)... I've also tossed some
inside shelters on bitterly cold nights. They do get expensive, of
course, but we've been able to get sporting goods stores to order
them for us wholesale. They are extremely effective, though I've
found you have to be careful not to cut off the oxygen supply to
them, or they just freeze.
I use Snuggle Safe discs for water, with limited success-- though I
would say they get the water unfrozen for at least a bit... what I
did was built a box for a water dish out of foam board insulation...
took one square of it for a bottom, then cut another square, traced
around the Snuggle Safe dish or water bowl (which ever is bigger),
cut a circle out of it, duct-taped the two squares together... then
I heat up the Snuggle Safe disc, put it inside the insulation box,
then put the water bowl on top. It looks ridiculous and I've never
stuck around long enough to see how it works, but it makes me feel a
little better... but you could probably do something similar w/ the
handwarmer things (microwave or disposable).
The Snuggle Safe discs are expensive, but they last a long time, so
not a bad investment. I have two, so I can swap them out as needed,
though sometimes if it's super cold, I wrap one in cloth and pop it
in the shelter...
One of these days I am going to learn about solar power and figure
out how to take one of those bird bath de-icers and power it with
solar power... it can't be that complicated, when they solar
fountains, solar flashlights, etc., etc...http://groups.yahoo.com/group/feral_cats/message/74297