I don't know of a list of sites using Gravatar, but I do know that many, many blogs use them. Apparently the company that made Wordpress owns Gravatar now, so Wordpress has Gravatar support built in. Also, here's a page listing a bunch of plugins that enable Gravatar, which may give a sense of which TYPES of sites most commonly support Gravatars.
http://en.gravatar.com/site/implement