One of my favorites is called "My Water Broke!"
You freeze little plastic babies into ice cubes. You can get the little babies with all the cake decorating stuff at most craft stores. You have to freeze them into the ice cubes in layers of water, kind of, so that they're in the middle of the ice cube.
Each guest gets a baby ice cube and they have to melt the baby out as fast as they can...(without using things like teeth). The first one to get their baby out will likely have a small puddle of water at their feet from the melted cube and has to yell "My Water Broke!"
ETA: I just read ollieorange's thread below with the same idea... so i guess I didn't offer anything new