Back in 2004, I implemented CAPTCHA checking on .NET 247. At the time I used as component called FormGuard from a company called Xheo (http://www.xheo.com/). In the intervening years, Xheo have stopped supporting this component, so I looked elsewhere.
In the end I found a link to a free component punted by a gentleman called Peter Kellner (http://peterkellner.net/2006/08/20/the-ultimate-captcha-custom-server-control/). Didn’t take long to get it working on the site and was a quick and easy solution.
One thing that was interesting during this process though is that if you Google for “captcha”, one of the links that comes up is from a company called “decaptcha”. They offer a service where they will defeat 1,000 CAPTCHA checks for you for $2. Minimum buy is $8. So – OK, I am an IT professional with a certain reputation to keep, but good grief, I presume this is one of these services where they pay a bunch of people in a room to read and defeat CAPTCHAs all day. What I found amazing about this is that Google are quite happy to allow them to advertise on their network…
And then on the subject of CAPTCHA, I love “reCAPTCHA” (http://recaptcha.net/learnmore.html), which is a service whereby in situations where people who are trying to digitise old books come across words that they cannot OCR, they farm out the text to people filling out CAPTCHAs – which sounds lovely in that your users end up contributing to community projects to scan old books.