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:
The “thin portfolio” concept (borrowing from the prior “personal information aggregation and distribution service” concept) represents the idea that you don’t need that portfolio information in one server; but that it is very helpful to have one place where one can access all “your” information, and set permissions for others to view it. This concept is only beginning to be implemented.
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.
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.
The ticTOCs Journal Tables of Contents service makes it easy for academics, researchers, students and anyone else to keep up-to-date with newly published scholarly material by enabling them to find, display, store, combine and reuse thousands of journal tables of contents from multiple publishers.
PostRank claims to measure social engagement, including blog posts responding to someone else, bookmarking an article, leaving a comment on a blog, or clicking a link to read a news item.
TubeMogul is a free service that provides a single point for deploying uploads to the top video sharing sites, and analytics on who, what, and how videos are being viewed.
Adobe AIR app that lets you display items in your RSS feeds in a scrolling ticker on any edge of your screen. Not for the faint of heart or the information averse.