How to Cut Styrofoam | eHow.com - Brief list of various methods
Hot wire foam cutter - Instructables.com: Step by step design using yardsticks for frame.
Foam Cutter - A more polished design using a nicely built spring loaded frame and a custom power supply.
Rocket Team Vatsaas Hot Wire Foam Cutter - PVC frame and a model railroad transformer.
YouTube - DIY hotwire cutter for styrofoam shaping - Using a car battery charger. Not pretty, but it works in the video.