I've not done a puzzle for a while and I heard this the other day.
Apparently a common Google job interview question.

You have 8 balls, all of which are identical expect 1 slightly lighter than the other 7. You also have a balancing scales. Using the scales only twice, determine which of the 8 balls is the light one.

Email me if you have a solution/question. I'll post the solution in about a week.

**Update:** I got lots of responses to this puzzle. The solution
is (I stole the best worded response, thanks Ian):

Keep two balls aside. Weigh three each in the scales. If the scales balance, it's one of the two spares. Balance them, and pick the lighter one. If the scales don't balance, compare two of the three balls from the lighter side. If those balance, it's the third ball.