ESBITs work really well.
There is a more rugged version folding steel stove that also contains the tablets - they were standard British army issue before chemical heating packs.
Not exactly precise control for fine cuisine and a bit pricey for extended use but half a tablet will make a cup of tea on a mountain in 5mins.

For all things Esbit http://zenstoves.net/SolidFuelBurner.htm

ps. If you are a Brit of a certain age - these are the fuel tablets from Mamod steam engines.