KCAPTCHA

This class can be used to generate CAPTCHA validation images with the verification text rendered with several effects.

It generates a random text using a list of allowed characters.

The text is rendered as an image with several effects to distort the characters and make it more difficult to guess by automated programs.

The text is rendered with fonts defined as images in the PNG format. So it does not need any PHP special font engine library extensions.

The generated image is served as the current script output either in JPEG, GIF or PNG formats.

LGPLで公開されています。フォントがpng画像として用意されているところが面白いですね。
スクリーンショットを見ても分かりますが、良い感じのcaptchaが作成できます。