Push Ups

There are a lot of variations on the classic pushup. Mark Lauren demonstrates some of these in this video:
 * Pushup progression -- from wall-pushups to declined 1-arm pushups (!!)

If you're doing inclined or declined pushups, a set of stairs can be a great place to work out, since you can vary the height of your hands or feet so easily.

Beast Skills has a great page on one-arm pushups.