Gravatar is a web service that lets users create profiles and avatars called Gravatars. A Gravatar is an image that follows you from site to site appearing beside your name when you do things like comment or post on a blog. Gravatar has several developer services that do not require application authentication (although to edit user information in XML-RPC a user must be authenticated).
Gravatar is an attempt to allow users to have one globally recognized avatar. The service, around for several years, allows a user can set an image as an avatar and have it recognized on a variety of sites. Later, users can change an avatar once and have it change on every Gravatar-enabled site that they use. Though the service is somewhat well known, what is not is its robust API for developers to integrate the Gravatar service into their own sites.