Social Graph API by Google

Google's Social Graph API allows developers to utilize the public connections among people on the web. The idea is simple, yet it could make it easier for people to connect across sites. Of course, the data all comes through Google, which yet increases our dependence on them. No doubt, other search engines could easily create the same API.

Since Google already saves a copy of all of the web pages that it spiders for search, the task of extracting the annotated links is somewhat trivial. Currently, this will only work as web developers annotate user links with XHTML Friends Network (XFN) and Friend of a Friend (FOAF). It is a great idea, but may take some time before web developers start annotating.
Furthermore, the easy access to people's connections is a nice data source for some new applications and experiments.