Whatever your reasons, I've compiled a fairly decent little montage of variations on the pushup you can experiment with. Some are suitable for people more on the beginner end of the spectrum, and some are pretty advanced. A few can be modified to do with knees on the ground if necessary. Your basic pushup form should be really good before trying most of these, so make sure you have that down. As with any exercise, if anything hurts or doesn't feel right to your body, DON'T DO IT.
Let me know how it goes, and if you have any questions, post 'em here!