I often see people getting excited about shape memory alloys, aka nitinol. At first glance it looks pretty neat: magic metal that predictably deforms when heated -- muscle wire for your metal robot!

While it may be great for a few specialized applications like medical stents, keep your expectations modest: it is too slow and rarely strong enough for most purposes. Moreover, it's very inefficient if you use electrical current to heat it, and will quickly deplete your batteries. Motors or solenoids are typically a much better bet if you want electrically controlled motion. Hobby servos are cheap and easily controlled with an Arduino or similar microcontroller.

Also, Nitinol gets burning hot if you don't carefully control current, so for anything wearable or touchable, that is not great. Given the high battery consumption, it seems not well suited for wearables.

I haven't seen anything more compelling than the "butterfly" demo, but some people are putting wires in parallel for more force (if not efficiency or speed), see these actuators from Sparkfun. Let's face it: if it was as amazing as it looks, we would see it everywhere small actuators are needed, and we don't.

Am I being too negative? Please prove me wrong! Do you know of a good application? Let me know!

Jameco carries some, including sample kits if you want to play with it.