Muscle Group: Chest
Pushups performed without weight add specific conditioning to the entire pectoral region because they are so high repetitions. Working on specific conditioning can bust you out of a plateau faster than you realize! In addition, if you place weight on your back they can also become a terrific mass builder.
Lay face down on the floor, then with your palms open, position your hands using a medium-wide position so that your forearms are perpendicular to the ceiling when the floor touches your pecs. (shoulder width placement)
Raise your self up so that just your pecs are touching the ground and nothing else, the rest of your body should be suspended. This is the starting position. Push yourself all the way up until your arms are fully extended, pause for a peak contraction and then lower yourself back down under control.