Check out www.crossfit.com
A lot of their workouts don't require any equipment.
A word of warning - these folks are crazy fit. Ease slowly into their workouts - do like 1/4 - 1/2 of what they suggest when you start or you will be incredibly sore and unable to function for days. Once your body acclimates, you'll start to amaze yourself. I've been on pretty strict excercise programs for years, and this stuff has given me results like nothing else.