Wall squats can easily be done as a work break -- all you need is a wall (and good knees). Wall squats strengthen the muscles of the seat.

Wall squats strengthen the muscles of the hips, legs and abs. Wall squats help strengthen the muscles of posture, especially sitting posture. Do wall squats!
(c) Anne Asher licensed to About.com 2006
Experts recommend that those of us working at our desk for long periods of time be sure to take mini-breaks to save our hands and back. The
wall squat is recommended by the American Physical Therapy Association.
Wall squats target the muscles of the hips, back, abddomen and quadriceps. They're great for sitting posture!