i think:

Any style that frames the face and reduces the roundness will work. Mainly hair that flows toward the face.

Thanks for all the suggestions!

Try a below the chin or shoulder length bob....the choppy bob on Scarlett in pic #2 would look great on a round face.

I would suggest layers and an aymmetrical bang. Here are some ideas:

Long hair is gorgeous on round faces because it lengthens the face. If you have a round face, your best cuts will fall a few inches below the chin and longer.
Ask your stylist to cut in long layers with the shortest layer hitting at your earlobe or even lower. Short layers make a hairstyle appear dated and also add body to hair, which will make a round face appear rounder. You don't want that.
Gentle waves deflect the roundness of the face, so if you have natural wave in your hair (as most women do), allow your hair to do its natural "thing." Scrunch hair as it dries so it retains some of its wave or help along the wave by wrapping hair around the barrel of a large curling iron as Biel's stylist did here.

Make sure to opt for a length that ends above or below the chin to help flatter rather than accentuate your round face shape. Also stay away from a full fringe as this really gives a more exaggerated roundness to the face. Instead, go for more of a side bang to help give off a more oval illusion.