It depends on how you do your pushups. If you want to build pectoral mass, archer pushups coupled with muscle ups will do nicely. If you have a medicine ball, then the 3-point pushups will also work well. Normal pushups must be slowed down in order to gain maximum benefit.

For martial arts training, we used to do pushups on a 2x4 plank balanced on top of a can of soup. The soup can was placed at quarter positions along the length of the 2x4. After a while, you were basically doing a one-armed pushup on an unstable surface.

edit:

Here's one of the books on the subject that I really liked:

http://www.amazon.com/Bodyweight-Exercises-Extraordinary-Strength-Johnson/dp/0926888781