Big, bright arrow-shaped wireless turn indicator (rear only)
Emits beeping sound, and small light on remote lets you know it's activated. Manual turn-off, no front lights, or brake lights, but looks good enough to alert people behind you. Not sure on the effectiveness during the daytime though. I'm also curious on whether it is waterproof.
My bike has a rack in the way, so seatpost mounting is out of the question for me, though I could probably figure out something.