relspider
relspider is a web crawler that indexes the identity/social graph of profiles.
The project was created by 
Pelle Wessman to crawl identities using rel-me links. It's focused on the identity graph subset of the social graph and on enabling similar queries on that subset as the Google Social Graph once did.
It started out as an experiment at Flattr to resolve verified ownership of web pages in an open, non-hands on way by linking authorship and identity to known verified ones such as the Flattr profile and connected third party ones like Twitter. When a link could be found a Flattr micro-donation to an author of some page on the internet could then be securely forwarded to the correct Flattr user without any kind of pre-registration of the content with Flattr. Unfortunately that project was never launched.