Whether you are posting comments on a blog or registering for some web service, they'll probably ask to solve some kind of CAPTCHA to ensure that you are a "human being" and not a spambot.
As users are very likely to read and type the CAPTCHA text, some people are suggesting to commercial this cryptic graphic. See good examples from Jean Yves.
Kiruba Shankar spots this funny Yahoo! India ad on his boarding pass. Waiting for flights at the airport is so boring.