There are dozens of free captcha generators that provide service or scripts to protect your site from automated spam posts, but not all generators are created equal.
Here’s a rundown of six different services and the different situations in which they can be helpful:

• AdsCaptcha combines Ads and CAPTCHAs, thus helping you not only to keep your site spam-free (and for free), but also make some money every time somebody solves that CAPTCHA. It’s as simple as it is genius.
• Captchator, for example is a free captcha service that requires a one of a broad base of coding languages to apply. You can run it if you have a PHP, Rails, Python, Coldfusion, Perl, or one of several other frameworks. It doesn’t require any extensions or libraries, and can be inserted with only a few lines of code.
• Snaphost’s Form-to-Email script is can be likewise inserted with a few lines of simple code and works with any HTML form.
• OpenCaptcha is a webservice that requires no special configurations or modules and has basic cut-and-paste installation, as well as new fonts, image algorithms and distortions applied weekly.
• Cryptographp is a PHP script that generates captchas. While script is free and compatible with PHP>=4.3.0. There are also plug-ins for WordPress, wpMu for WordPress, Symfony, and Guppy.
• freeCap is a GPL captcha script. It requires some knowledge of PHP to install, but there are several plug-ins for forum and blog software available.