Crank Brothers make the Eggbeater Pedals which come in different styles such as Candies, Mallets etc. The original Eggbeater is a small, 4 sided pedal which frankly looks like an eggbeater or whisk (you know like to cramble eggs with), Candies (as shown above) have a small platform which surrounds the eggbeater pedal, Mallets have a very large platform (and are suited for mountain biking).
The amount of surface supposedly distributes force outward. Many people like the smallness and ease of the original Eggbeater, plus it's very light weight. Platforms add weight but you gain some stability.