In downtown Seattle's Metro Tunnel at University Station (3rd & University), there is an art installation--"Saccadoscopoeia" by Bill Bell--on the northernmost wall of the mezzanine. It consists of scattered red and green columns of flickering LEDs, spaced evenly left to right.
To a casual observer, it looks like nothing more remarkable than randomly flashing vertical lines. But by standing back a distance, focusing on the display, and quickly glancing back and forth, you can use the persistence of vision effect to see hidden images being flashed on the LEDs.
These images share a theme that changes about every 1-2 minutes. On one afternoon, for about 10 minutes, I watched these themes go by: stock market terms, crabs, sea vessels, fish, and the old (pre-King County) Metro Transit logos.
(From a post I made to the Easter Egg Archive <http://www.eeggs.com>)