Everyone has heard about the guy who attempts to measure pi by throwing pins on flags and estimating the probability that it lies entirely in one stripe (Buffon's problem).
Here I would like to add a few more serious techniques for finding Pi, just in case you find yourself stranded on a desert island without a computer or a copy of Pi to a Million Digits (one of my all time favorites for light reading).
Of course if the island is perfectly circular the problem is trivial.

Literary Methods

First you can count the number of letters in the words of the following: The last is has the nice feature of having a decimal point in the right place. It is also almost optimal since it is followed by 50288, and it is hard to come up with pronounceable word with zero letters (and the poem has grown wierd enough as it is).

Scientific Methods

There are various experiments one can perform that will easily return pi.

Less Scientific Methods