NetNewsWire is a popular RSS aggregator on the Macintosh. NewsGator is my favorite aggregator. NewsGator already own RSS aggregators on:
1) Outlook (their original client).
2) Windows (FeedDemon, which they acquired earlier in the year).
3) The Web (they have a pretty comprehensive service that you can read, and rate, in your browser).
4) Windows Media Center (getting a LOT more important with HDTV and Xbox 360).
5) SmartPhone and PocketPC through Web service.
(Via Scobleizer: Microsoft Geek Blogger.)
This is good news, I love NetNewsWire!