Identicons from Gravatar
ma.tt/2008/04/identicons-from-gravatar
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
You may notice in my comments now for people who don’t have an existing Gravatar I show cool geometric patterns. These are called Identicons and they were originally conceived by Don Park. With a single parameter, you can have the Gravatar API fall back to an identicon or even force it to return one. This works in any size Gravatar supports, up to 512 pixels. Sweet!