There is this one machine that really works the butt, you lay on your tummy and lift a lever up with your calves. The men at my gym LOVE this machine, I spend an extra 20 minutes at the gym just to use the darn thing.  Now with the resolutioners already clogging up the gym, every one uses this machine and I can't even touch it without hearing some shrill "oh I was next!" from some shamu jr.  Die? Die.