'Are You A Human' is an interesting alternative for standard CAPTCHA systems. Instead of using text-based CAPTCHA which can be unpleasant for users, it uses dynamic games (PlayThru™) which are harder to crack but easy for humans to solve.
http://www.wysiwygwebbuilder.com/areyouahuman.html
You can download the new extension through the Extension Manager (Menu->Tools->Extension Manager->Available).
Please read the help!