The XOOPS captcha system was reportedly hacked and no longer doing its proper protection from spam bots (nor did it offer a version for sight-impaired folks). A community hack was released for how to replace it with reCAPTCHA, now a Google project.

You essentially just have to register and get a key for your domain. Then download the reCAPTCHA php library and implement the code changes as outlined in the XOOPS reCAPTCHA hack. Perhaps this will be used in XOOPS 2.5.x and beyond. I hope so.

I found it easy to implement. Apparently for XOOPS 2.3.x+, I implemented first in 2.4.5.