If you own a website, a forum or a blog which requires users registration, and are anxious to block those endless spam bots – you certainly need Captcha.

Captcha is a small security image of distorted letters, and/or numbers, that is supposed to be copied by users in order to distinguish them (human users) from computers so the unwanted bots could be filtered out.

How can you use Captcha on your site?
You can buy captchas or get it for free, using an open source Captcha.

Following is a small list of Open Sources Captchas,
free resources for Captcha creation and/or implementation:

“Simple Captcha” is an open source Java  implementation for creating Captchas.  It gives assistance in generating good looking Captchas with a minimum of programming and little configuration.

JCaptcha is another open source Java framework for Captcha integration and definition.

“Securimage” is an open source Captcha script, for creating complex  images and Captcha codes. It can be easily added to an existing form on the website, and can run on most web servers as long as you have PHP installed. It generates Captcha images, it validates the typed codes, audible codes, it does everything!

“Joomla” is an open source CMS that has free Captcha plugins.

Plone Captcha is a free service for Plone captchas implementation for validating human input in online forms.

Later Addition:
# ASPxCaptcha – check this post to watch an introduction video on how to setup the ASPxCaptcha control on your site.
# Check this post for some easy instructions on how to implement captcha into cakephp.