Lego Mechanics/NXT 3-axis CNC Prototypes
I'm a huge Lego fan and often finds it creatively insightful to prototype product ideas in Lego form. As a prototype medium, it's fantastic. It's easily adjustable in size and function. You have programmable functionality via steppor-motors, DC motors, all kinds of sensors - ultrasonic/distance, color measuring, signal transmitters, push sensors and more.
What more do you really need? I love Lego.