The Brachistochrone problem. Solved by both Newton/Bernoulli. It is considered to be the fundamental/first problem which led to the formulation of the Calculus of Variations.
Find the shape of the curve down which a bead sliding from rest and accelerated by gravity will slip (without friction) from one point to another in the least time.
alt text http://upload.wikimedia.org/wikipedia/commons/d/d4/Brachistochrone.png