Matthew Cosier has working on an API to stream and interpret ATOM 3 and RSS 2 feeds.
You can easily build an aggregator from this becuase you can read multiple feeds in one time before analysing them.
Should be released soon but I also recommend Marcus's tool B.ag, used on Developers.ie for the blogs.
Interesting to see that web aggregators are back in force, instead of using a proprietary application.