Gravatar

 Gravatar  is a silo service run by Automattic to host a profile photo (avatar) for e-mail addresses. The photos can then automatically be displayed on your posts and comments on any website that supports it. Some sites use the service to also provide hovercards with additional identity information.

Criticism
Gravatar is based on MD5 hashes of e-mail addresses, which has been used to deanonymize users in the past:,. You're effectively leaking weakly-hashed user data.