cool-php-captcha is a cool PHP library for generating captchas.
Sample code:
session_start();
$captcha = new SimpleCaptcha();
// Change configuration...
//$captcha->wordsFile = null; // Disable dictionary words
//$captcha->wordsFile = words/es.txt; // Enable spanish words
//$captcha->session_var = secretword; // Change session variable
$captcha->CreateImage();
