Josef Prusa made an innovative ultra-low cost automatic printbed leveling mechanism for his RepRap.
It is made out of two glass plates and four micro switches (one in each corner) activated by printhead pressure.
The setup costs some 20 cents, so it is probably the easiest to make and cheapest system around.
Only limitations are that the switches get pressed if you print an object heavier then 300 grams and that you are limited in using heated bed.
