Unlike similar implementations, this PHP CAPTCHA class works without cookies. It is extremely easy to integrate into existing sites and your <form> processing logic. And it requires no database.
It strives to be more user-friendly. It gives visual feedback while you enter a solution (AJAX). And it accepts a few misguessed letters. Additionally it has an accessible riddle built in, optional modules and many configuration settings.
Following is a pseudo blog comment field - only here for testing:
Distributed here as Public Domain, which makes it compatible and convertible to all open source licenses.