BOTCHA lets spambots to prove they are bots, and let real users zip by.

BOTCHA is useful for any form that has to be protected from spambots.
How it works

The approach of BOTCHA is to add various elements to forms that need protection from bots. These elements do not present new fields to users, so BOTCHA is completely transparent to humans. Both humans and bots submit those forms and BOTCHA performs heuristic analysis on each submitted form. Bots are usually programs/scripts that are relatively dumb, and most of the time they fail BOTCHA tests and human users don't.

Once BOTCHA proves the submission is by a bot, the form submission is blocked.