Social whitelists
I’ve talked about these previously; Tom Coates has further thoughts. By sharing whitelists we can use OpenID to build a simple trust network.
A similar concept is that of publishing groups. Jyte offers a simple API to export the members of a Jyte group. Not only does this make groups portable to other services, it also lets you build an authentication mechanism for a site that only allows members of a specific published group to log in to a service.