The other day I came across a site that was using the reCAPTCHA CAPTCHA. I'd heard of the project but hadn't had time to investigate.
The reCAPTCHA project (of the School of Computer Science at Carnegie Mellon University) provides both text and audio CAPTCHAs via a web service. It uses words scanned from books in the Internet Archive that have failed to be recognised by OCR, so using this on your site is helping to digitalise those books. See http://recaptcha.net/learnmore.html for more info on how it works.
With the Christmas break and the fact that we've had an intense low hanging about for the duration (strong winds & rain) I thought I'd check it out and see how easily it could be integrated into eZ Publish.
Turns out it was pretty simple. I've created an extension with a reCAPTCHA datatype that can be used in both regular content class (for forums, comments etc.) and as a information collector (feedback forms etc.). The datatype doesn't store any information but utilises datatype validation methods to check the CAPTCHA and return a pass or fail.
It's an initial version and I've only tested on eZ4.0. Hopefully I'll be able to test on earlier versions in the coming days. Mapping of languages also needs looking at.
For those interested it can be downloaded at http://ez.no/developer/contribs/applications/recaptcha