i@m.fm

You know these graphics where you have to re-type a phrase or some
characters or number in order to prevent automated registrations or
queries:

Boing Boing: Solving and creating captchas with free porn

Basically the spammers make porn-hungry people work. So they show them
the so-called ‘captchas’ from other sites and ask their site visitors
to ‘crack’ the code for them in order to get past the protection. Just
like a human brute force attack.