Securimage’s a quick solution to adding a Captcha to your projects. I liked the ReCaptcha project which provided you a hosted solution for captchas but, this one is a PHP and GD solution which you can run off your server.

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.

Some of the key features of Securimage are:

• Ability to add a captcha to your forms in 3 lines of code and validate the submitted entry in 6 lines of code
• You can create an audible Captcha in wav format
• TTF font support
• Customizable Captcha code length

To get going using Securimage, download the package and head over to their quick start guide.