SimpleCaptcha is a Java library for generating CAPTCHA challenge/answer pairs. SimpleCaptcha is intended to be easy to implement and use sensible defaults, while providing easily-accesssible hooks for customization. Example implementations are provided in J2EE and Grails.
Some sample CAPTCHA images which were generated by SimpleCaptcha:
Visually-impaired users can be given audio CAPTCHAs:
Audio Example 1
Audio Example 2
Internationalization is supported, with the ability to generate Chinese and Arabic image CAPTCHAs provided out-of-the-box.
SimpleCaptcha is open source software under the BSD License.