A lot of good answers to this question:

My four favorite "solo drills" for footwork/head movement:

(1) Shadowboxing - bar none the way I figure out new techniques and footwork (the walkoff and crawling are two things I'm working on now)

(2) The double end bag - already mentioned, but great. Teaches you to hit a moving object, and react with with movement towards you by moving your head.

(3) The slip bag - an Iron Mike favorite. Great for working slips and counters.

(4) Bobbing and weaving under a rope - not great for MMA, maybe. Other guys may be able to answer that with more conviction.