Often overlooked, I think bodyweight exercises are under-rated in today's training, maybe not so much for powerlifting, but from a bodybuilding perspective. When time or location does not permit weight training, body weight exercises provide a simple and effective means to grow. This is my experience from being in the military. I'm asking everyone to submit their favortite/ most-challenging bodyweight exercises in hopes that I, along with others, will learn some useful stuff.
My favorite: Handstand press. Do a handstand and lower yourself til your head touches the ground, and then push back to the start. Use a wall for support, and work up to doing them in the middle of a room. Then go bang the horde of chicks watching you.