A Jetboil is great kit for boiling water quickly, and can even be held in the hand if you don't have a level or stable surface to work on, but it runs off gas.
As for a quick brew on the go, I recommend vacuum flasks - you can get tough metal ones that survive a bit of abuse and keep hot stuff hot for quite a time now, you can't get quicker, and if you just fill with water and brew up in the cup (rather than filling whole flask with tea or whatever) you don't even really need to worry about cleaning it. I've a little 330ml pocket one which I do fill with tea or whatever, cross between a cup and a flask really, but 0.5L and 1L are probably better if you're working out of a wagon or leaving it in the car.
Myself, I also have a Hexi cooker in the back of the car amongst other bits and bobs, the rationale being it IS a bother to set up and clean up after, that way I'm not tempted to use up all it's fuel by brewing up every time I have a spare 5 mins, which means it should still be there ready for use when I really am stuck for hours someplace cold and remote. It's like emergency rations, better make sure it isn't too tasty, or you just know it will end up getting munched by someone way before it's really needed. Just a thought, there are worse ideas than keeping a cheap, small, simple backup burner, just in case your everyday one should suffer a problem.
