I've created an alternative to CoolAcid's OSTCaptcha because my visitors thought it was too hard to read his captcha. In theory, it will work with every extension that uses OSTCaptcha without need of code changing (i said, "in theory").
Bigo Captcha is a native 1.5 plugin and uses animated captchas to keep spam bots away from your site. It also has a protection that avoid "guessing" from spambots, so If your visitor tries to guess 15 times without success, he/she will be ignored until next session.
I've tested this extension with yvComment and it seems to be working pretty well.
Download / Demo / Price (

http://extensions.joomla.org/component/ ... Itemid,35/
Use this thread for suggestions and comments. If you use my captcha, please write a review.