Mountain Dew, at least that's what it looks like...
No, really, its an alcohol based coolant. Some may even use real ethelene glycol (like in cars). They are sealed though. They really have too many lines and surface area to be practical...for now. I know the computers in our patrol cars have some kind of heatsink on the rear that feels like a metal, but it really pulls the heat out, almost as if it were active, but its not. They do have some materials, exotic to be sure, that can really pull the heat. Alloys of some sort.
Ethylene glycol? Doesn't flow that well, and doesn't beat water for heat capacity (few things do, actually). It's good as addition to prevent freezing though.
Most hobbyists use water (with some additions some claim have magical properties
) and serious professional stuff are usually some kind of fluorocarbon thingy.
For a nice neat light though, i'd go with an old computer heat sink, add a fan, and run it. Once you get the heat off the LED(-array), it's easily dealt with.
Using and focussing the light would be the problem, as larger arrays are hard to form into a nice beam (see the reflectors on multi-die-lights), and using it as a floody doesn't attract me.
I'll stick to a nice MC-E for the time being
@Yellow: This thing is bigger (6x6mm instead of 4x4mm of an MC-E), eats 12V @ 6A, and some more differences