My yoga teacher loves the Buddha squat pose. Basically a squat with feet FLAT on the ground and butt and hips below thighs...she likes to hold it for what feels like hours. I usually think I am going to die about 2 seconds before we finally move out of the pose.

My rugby coach used to make us do "squat thrusts"...start in push up position, jump into squat with feet between hands, jump straight up as high as you can, land in another squat, jump legs backwards into push up position again. Sometimes he'd make us do actual push ups between the squats. His strategy worked though. Since rugby consists of two halves with no breaks and no substitutions except in cases of severe injury, we won a lot of games solely by being fitter than our competition...