I built a low-cost gyroscope using a slotted optical switch, paper that I printed different bands of gray and black, Lego and gravity. I saw a slotted optical switch used for a tachometer, so I expanded on that idea. It's only 2 dimensional and needs dampening, but you get what you pay for.
details: http://underfinished.blogspot.com
video: Four Dollar Gyroscope - YouTube
Cool, I can't imagine it's all that accurate, but you get what you paid for!
Let's see, with my many tubs of lego, and lots of money saved up that I hardly ever spend, I could build:
A B-DOF gyro board!*
P.S. not to be nitpicky, but isn't this functionally more like an accelerometer?
*bajillion DOF
Actually, it seems to be working more like a tilt switch except that it is slightly variable. 
Cute idea though. That's how new products get developed.