Every one has used CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) or HIP (Human Interactive Proof).
Captcha is usually numbers and letters and the more obscure they are to prevent programmatic interpretation, the more difficult to read.
Microsoft Research has an interesting alternative.
Asirra (Animal Species Image Recognition for Restricting Access) is a HIP that works by asking users to identify photographs of cats and dogs.
You can get a complete description and implementation guidance at http://research.microsoft.com/asirra/.
But remember, it’s still beta !