From Citizendium, the Citizens' Compendium
- Please sort and annotate in a user-friendly manner and consider archiving the URLs behind the links you provide. See also related web sources.
- Official website
- reCAPTCHA, a common form/example of a CAPTCHA
- A paper on breaking a CAPTCHA
- Re: CAPTCHAs – Understanding CAPTCHA-Solving Services in an Economic Context — a detailed analysis of the market for solving CAPTCHAs