RoboHash
robohash.org
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.
Robohash is a easy web service that makes it easy to provide unique, robot/alien/monster/whatever images for any text. Put in any text, such as IP address, email, filename, userid, or whatever else you like, and get back a pretty image for your site.