The big example is the Facebook stream, which is based on FriendFeed. Facebook has spent a lot of time optimising the finer details of elements in its streams, from the spacing and positioning to the microcopy (like/likes, person/people, etc). I recommend you look there for the biggest patterns. Try posting a few updates of different types into your stream to see how Facebook handles them, eg. video/photos/a single photo/text/quotations/link to a website/poll, etc. If you pay close attention you'll start noticing how things subtly change depending on various invisible factors like how many people like something or from who the update is.
Google Buzz is also exploring a lot in this direction, but I don't use it much so I haven't paid as much attention. You can also use Buzz to compare to Facebook and see which patterns Buzz has "copied" simply because they've become conventions. Try referring back to Friendfeed itself to see where some of the ideas for visualising aggregation come from.
A completely different take on aggregation is Twitter, which refuses stubbornly to pollute your follower feed. Instead, #newtwitter leaves things as they are but allows you to click on each item and view a popout that might do things with enriched content like videos or images. That's another way to approach the problem if your priority is keeping the feed clean, or as in Twitter's case, being platform agnostic and keeping your overhead low.
The abovementioned are the leaders in this arena IMO and the ones you want to pay attention to for design patterns and conventions. Most other designers will be doing the same.