By far, my favorite sculpting medium is lightweight, air dry clay. I could play for days on end with just this stuff and a few paints. In fact, that's exactly what I've been doing!
These little characters (numbers 54, 55, & 56 if you're keeping up with my 365/2013 Character Creation project) are samples for that sculpture class I am teaching on Fridays through May at a local school. The sculpture class is presented in sections divided by medium: wire, found objects, paper, and clay.
These character busts are built up over a ball of tin foil that is taped to a recycled pudding or yogurt cup. I layer on the clay a bit at a time, smoothing as I go and then begin to add features.
One of the things I adore about scuplture is that I never know ahead of time what characters are going to emerge. Obviously, I could do preliminary sketches and then tailor my sculpting to produce a reasonable 3-D copy of my drawing. I prefer, however, to just let things develop serendiptiously. It is much more fun that way!