Tough AI problem -> game -> surfers will solve for free!
espgame.org
espgame.org
I'm curious, why do you think that visual games are more likely to be successful? His question answering games seemed useful and engaging, too.
There must be great startup ideas lurking behind this idea of games and Human Computation. (Luis alludes to as much, with his tongue-in-cheek suggestion that the ESP game double as a dating service.)