I keep all sorts of calendar info in the web, as do my colleagues and the groups and organizations we participate in.
Suppose it was all in RDF, either directly as RDF/XML or indirectly via GRDDL as hCalendar or the like.
Wouldn't it be cool to grab a bunch of sources, and then tabulate names vs. availability on various dates?
I would probably need rules in the tabulator; Jos's work sure seems promising.
Closely related is the PathCross use case...
Suppose I'm travelling to Boston and San Francisco in the next couple months. I'd like my machine to let me know I have a FriendOfaFriend who also lives there or plans to be there.
See also the Open Group's Federated Free/Busy Challenge.