evlos

Cache gravatars for you to speed up the loading. It works perfectly.

WP Gravatar Mini Cache

Cache gravatars for you to speed up the loading. It works perfectly.

Julio Potier

This plugin takes place between the gravatar picture and the default avatar return. If an email is not associated with a gravatar account, a Google Image request is sent to grab the first best relevant face picture of this member.
When a google image photo have been chosen, the user will receive a mail with 3 links, 1st to delete this avatar if not relevant enought, 2nd to refresh it, if its email is not associated with a gratavar account, 3rd an invitation to create a gravatar account.
Also, you can add you own avatar by uploading your picture, easily !
Then, you can use a gravatar cache to keep all avatar files on your server and, limit the HTTP request on gravatar.com
Here comes a (french, just watch ;p) demo video, 15mn length. (video = version 1.8 !) 
http://www.youtube.com/watch?v=90mBwyNd5zU

Gravatar Google Image

Add a step between "no gravatar account" and "default gravatar sent back", using Google Images Search Service + 2 bonus usage

WP Gravatar Mini Cache vs Gravatar Google Image