This plugin generates user icons using the wavatar algorithm.

Wavatars are small avatars built out of a set of available parts. These parts are selected by first taking an md5 hash of an input string (in our case, the registered email address of the user in question) and using different bytes of the result as the selector.  The wavatar algorithm was originally developed by Shamus Young as a WordPress plugin and originally available here:

http://www.shamusyoung.com/twentysidedtale/?p=1462

This plugin overrides the default user icon for users who have not uploaded an icon. A configuration option is available for those who want to use their wavatar instead of their existing icon, available from the icon editing page.