Vinu Thomas has posted about a PHP/GD CAPTCHA solution he's come across - Secureimage.

From the company's description:

Securimage is an open-source free PHP CAPTCHA script for generating complex images and CAPTCHA codes to protect forms from spam and abuse. It can be easily added into existing forms on your website to provide protection from spam bots. It can run on most any webserver as long as you have PHP installed, and GD support within PHP.

Features of the tool include simple addition and validation methods, the ability to make audible CAPTCHAs and True Type font support. You can download the software from the project's website.