<p>I'm in the process of adding CAPTCHA validation to one of my websites and need to know what open source solutions exist.  Please note strengths and weaknesses and what platform they work with.  I'm primarily interested in ASP.NET solutions but feel free to include PHP, Java, etc.</p>

ReCAPTCHA
is the same one StackOverflow uses. It has an
ASP.NET implementation
.
It uses a webservice to provide the captcha images; this is an university trying to digitize difficult to OCR texts. I'm not sure how easy it would be to replace this portion. There are many
open source client implementations
, as the API is
quite well documented
.