A new, jokey-but-cool proposal for reducing automated signups uses kittens instead of scrambled text to keep bots out and let humans in. CAPTCHAs are tests that try to figure out if a web-page is being accessed by a program or a person by requiring a task humans are better at solving, like typing some distorted letters into a box. They're often breakable with code, and when they're not, they're breakable with porn.
The KittenAuth test puts up a mosaic of cute baby animals and asks challenges the user to click on the kittens. This is easy for sighted people, pretty hard for software, and a little too incongruous to be a likely candidate for porn-based CAPTCHA solving. Ingenious!