The reCAPTCHA system provides a very good captcha system based on scanned books. See Google reCAPTCHA page. You need to obtain API keys to use the service, which is freely available for personal use.
The KeyCatcha system provides a captcha system based on JavaScript functions to reassemble a picture. See KeyCaptcha page. You need to obtain an API key to use the service, which is freely available for limited use.
The text captcha constructs easy text questions. They can be broken relatively easy.
The image captcha constructs obstructed images using Python imaging library.