Stoves that can be used in an emergency situation for cooking, and possibly heating.
Alcohol stoves have one huge benefit over other stoves — they can be safely used indoors, which means they make it much easier to cook, AND they can be used for heating.
Campers / hikers aren't as keen on them because the fuel doesn't provide as much heat for the same weight of fuel, but when you're at home, fuel weight doesn't matter.
Buddy burners are useful because they're stable and inert, and can be stored almost anywhere. They're not as good as an alcohol burner IMHO, but it may be useful to some as a backup.
How to make them: 
Wood stoves have the upside that the fuel is plentiful and easy to find. Downside: you have to cook outside, and so they can't be used as a heater either.
Science and theory:
Specific models I like: