In Activity Streams, verbs are their own objects, and the variety of actions that can be represented is limited only by the standard itself. Providers can also use verbs outside the standard, taking the chance that they'll eventually be incorporated, or that a downstream client could parse them anyway. Here's a list of the verbs incorporated in the Activity Streams standard so far:
APML allows users to share their own personal Attention Profile in much the same way that OPML allows the exchange of reading lists between News Readers.
Love Digg's RSS feed but don't have time to keep up with it all? Disstill is like your regular Digg RSS feed but filters out stories below the minimum diggs that you set.
By filtering your results by popularity, you'll be able to pare down a bunch of results that are presumably all relevant into the top sites on your topic, or some surprising ones you might not have heard of.