TGCaptcha
This is a TurboGears (http://www.turbogears.org) widget project.
You can view the widgets in the Toolbox.
This version has a nonce-system: when initializing the Validator,
you can pass a function which takes a single argument: the nonce
that's used.
This function should return True if it has never seen this nonce
before, and False otherwise.
For additional information and documentation, please visit:
http://code.google.com/p/tgcaptcha/