You know how waterbeds have to have heaters, or they'll suck the heat right out of you?
Soothsoft has found a way to harness this awesome power for good. They sell pillows that you fill with water-- they'll stay nice and cool for a while unless the weather is really rotten. In that case, they can be cooled farther by refrigerating them.
I'm very pleased with mine, but to be fair, the amazon reviews vary from love it to hate it.
Soothsoft has an awful website, but there seems to be a standard chillow, a slightly larger chillow (with velcro?), and a small pet-sized chillow.