Tricep Dips

Sit on the edge of a chair with your back straight. Hold on to the front edge of the seat making sure your knuckles are facing forward.

To make it easier try bending your knees with feet flat on the floor.  To make it more difficult; either have your feet on the floor as far out as possible, or place another chair where your feet would be and place them on it (backs of your heels).

Push your butt off the seat using your arms (making sure not to lock your elbows) and then lower your butt until your arms form a 90% angle and repeat.