Any cyclist who's had their fair share of close calls with an errant motorist while travelling in the bike lane will appreciate this concept out of the University of the Arts in Philadelphia.
Called Biker's Revenge, the design is quite simple, and would definitely be effective from the looks of things. Personally, I won't believe these are real until I see substantial unedited video of creators Matt Braun and Jared Delorenzo wreaking havoc on the streets of Philly (and the subsequent arrests).
Anyway, the concept is simple enough. It's pretty much just the bottom halves of two keys; attached to plungers with a dash of malice; which you then insert into the hollow part of a bicycle's handlebars while cackling maniacally about the evils of society.