Seeing the Flip Clock offered now by ThinkGeek immediately took me back to my childhood and reminded me how fascinated I was with this type of time display.
The numbers of the time display fall in place with plates that rotate against a bent metal finger that releases them only when it is the next plate's turn to fall. What really got me wondering as a kid was if the plates just had half of the number printed on them. When falling the plate revealed behind fits the backside of the fallen plate - magic!

The retro flip clock is battery powered and features an alarm function. It requires 2 "C" cell batteries and measures 175 x 60 x 90mm.
The Retro Flip Clock sells for $59.99 on ThinkGeek.