ActivityStreams is the next big thing TM. It defines a protocol to syndicate activities taken from web applications (mainly social ones) and services. Just imagine a common spec for describing the activities a user do on Facebook, FriendFeed, Delicious, etc.
The protocol originally targeted the Atom XML Standard, via an extension. Later, some work has been done on standardizing a JSON format too.
Here’s a simple demonstration on how it works. We take the excellent Cliqset feed proxy and get an activity streams annotated Twitter Atom feed:
The JSON API is very similar. Some documentation is up on rdoc.info and it is worth checking the spec too. Have fun :)