It doesn't matter how much protective gear you wear or how bright your safety lights are, on a bike, cars consider you an afterthought at best. But with this homebrew bike horn, people will know you're coming from over half a mile away.
The brainchild of Instructables contributor The Chadster, this system mounts a 10-gallon air tank on the rear of the bike with dual air horns on the handlebars. An airgun attachment acts as the trigger and hearing protection is recommended.
Altogether, assembly takes about a half hour, though parts cost an estimated $200-plus. The Chadster recommends only employing the horns to terrify pedestrians rather than startle motorists but, c'mon—that guy triple-parked in the commuter lane totally deserves a toot. [Instructables via Technabob]