Flameless heater kit on sale at REI
I'd never seen this before, but I was looking through the REI sale items and found this. It says that it will heat 12oz of water to 146 degrees in 10 minutes.
It requires carrying heating packs, which are about $15 for a pack of 7, and doesn't mention whether the used packs are considered HAZMAT (the MRE heaters are), which would require you to carry them out with you.
Oh, and if they're the same chemical composition as the MRE heaters, you can't use them in a tent, besides the nasty smell, they create hydrogen gas as a byproduct.