Stack Overflow uses Recaptcha to dissuade bots. Recaptcha provides an audio test for users with difficulty reading.
Accessibility. CAPTCHAs must be accessible. CAPTCHAs based solely on reading text — or other visual-perception tasks — prevent visually impaired users from accessing the protected resource. Such CAPTCHAs may make a site incompatible with Section 508 in the United States. Any implementation of a CAPTCHA should allow blind users to get around the barrier, for example, by permitting users to opt for an audio CAPTCHA
However, try one out http://www.google.com/recaptcha/learnmore or http://stackoverflow.com/captcha. They are impossible! Not merely difficult— every single one is impossible! Google (who develop Recaptcha) are delusional to think this is accessible.
This is what it played me: http://picosong.com/w6Fs/