I have been experimenting with various designs of improvised camping stove for some time now. They have varied from a simple open cup of burning fuel to highly intricate vapour pressure devices that take hours to build.
This is my latest favourite, it is a combination of two established types of stove. A chimney stove and a low pressure side-burner. It takes about three minutes to make and so can be constructed as and when you want to use it.
This design also has the advantage that it will run on pretty much any flammable liquid you put in it. I have used methylated spirits, rubbing alcohol, xylene, white spirit and petrol.
This brings up an important safety issue though, anything other than an alcohol type fuel is inherantly a lot more dangerous. Pure hydrocarbon fuels like xylene and petrol are toxic, dirty (they make a real mess of your pan!), smelly, potentially explosive and can't be put out using water. An alcohol fire can be extinguished by upending your pot of water over it if things get out of control (don't believe me? Try it.)
For more info on improvised stoves have a look at the site that inspired me to try experimenting: zenstoves