The Wikipedia page http://en.wikipedia.org/wiki/R_v_Dudley_and_Stephens is very interesting; it describes an actual case where one of the people in a lifeboat was killed and eaten by the others, who would have otherwise all died. They claimed it was a "custom of the sea" that you could draw lots and kill one victim in such circumstances, but the court decided in a highly controversial case that necessity is no defence against a charge of murder.

An amusing article about the trolley problem is "Brain Teaser" (http://www.mindspring.com/~mfpatton/harper%27s.htm) which satirizes these ethics problems by adding an absurd number of conditions such as a busload of orphans, transplant patients about to die, a brain in a vat, and the inventor of the pop-top can.