The captcha.php library is very user-friendly and extremely easy to integrate into existing forms, because of its simple API. It nevertheless sports many configuration options, like multiple image modes and accessible alternative text riddles. It also has "AJAX" super cow powers and many sensible defaults to frustrate users less than other implementations. It prevents spambot access well, even with its relaxed defaults.
CAPTCHAv2 is a question/answer based CAPTCHA system. It has been designed to protect any Web form from spam bots attempting to submit information automatically by asking questions the visitor needs to answer. It can also detect brute force attempts and will block further abuse by adding the visitor's IP address to the .htaccess file. CAPTCHAv2 prevents accidentally blocking good bots by verifying the whois record of the visitor's IP. It is also possible to use CAPTCHAv2 as an automatically blocking bot trap. Bot trap implementation guide can be found in the guides.
image_captcha will pull the images out of a user-defined directory and obfuscate them before rendering them to the screen. It will then allow the user to select the correct image via a select box or a radio button. This also allows the admin to choose the number of random words or filenames that will appear in the select box or radio field. Thumbnail support is available, in which missing thumbs are automatically generated from larger images and then displayed. Audio support is also provided for the visually impaired.
RainCaptcha is a CAPTCHA class that does not require any image processing extensions (GD, ImageMagick, etc). CAPTCHA was developed to be readable by humans and resistant to OCR software. It generates black-and-white images with 5 distorted letters on them and noise. Its checking algorithm is case-insensitive.