One possible candidate to evaluate https://github.com/Gregwar/Captcha

Instead of just replacing one captcha solution with another, I think it would be great if captchas could be modularized and customized. I created 0021863 for this a while ago. The main rationale consists of two points:

• Being able to offer e.g. a text-based captcha would increase accessibility.
• Custom captchas drive away spammers, as they usually concentrate their effort on the default captcha methods.

To satisfy both points, a custom captcha could for example consist of user-defined textual questions, which typically cannot be solved by spam bots, while also being accessible to blind users.