Gravatar has been around longer and Openvatar might be kind of redundant but the concept sounds pretty cool to me. Note that WeeWorld (and others too I’m sure) have offered a similar service for even longer than Gravatar, but there’s some key differences.
Gravatar lets you upload or link to images you want to use as an avatar and then associates them with your e-mail address (you can specify multiple e-mail addresses and use the same or different avatars for each one). So now sites that display your avatar can request the appropriate image from Gravatar based on the e-mail address you used at the original site! And you manage your avatars in one place: Gravatar.com.
If the site doesn’t have Gravatar integration but supports linking to an off-site image then you can get the image URL for e-mail address(es) post them to the site.
This latter feature works just like Wee World’s live linking to your Wee Mee. As you change your gravatar (or WeeMee) the image generated by the link will update.
As for Openvatar, the service is redundant since OpenID allows transferring an avatar (or profile image) but since there are a number of OpenID providers the chore of managing your avatar at each provider could be simplified by a service like Openvatar (or by Gravatar extending their service to include your OpenID’s). I realise the promise of OpenID is that you need only ONE but the reality is you’re going to have several.
The wonderful thing about the internet is all the standards you have to choose from!