related: Sensor Technology, Human Interface System

• electric motor
• brakes
• pneumatic (air)
• hydraulic (water)
• Shape memory alloy (metal, as lina described)
• piezoelectric
• piezoelectric pressure valve:

http://www.acx.com/lab/cool_piezo.html

Heat dissipation and response time may be significant impediments.

Sample kits available through hobbyist sites.

By the meter, this stuff is rather cheap (£5 per meter, roughly)

http://www.dynalloy.com/sampleapplications

http://www.smst.org/smstweblinks.html

http://www.robotstore.com

Including numerous kits and a text: “Actuator Design Using Shape Memory Alloys”, as well as off the shelf integrated shape memory alloy actuators (20 cost 8000¢, or 100 cost 300 USD) These have 20 mm displacement, and one pound pull