Feedreader is a lightweight open-source aggregator that supports RSS and ATOM formats.

Feedreader Features
Possibility to combine multiple feeds to one feed,
Speedier handling of large number of feeds
Powerful filtering and smartfeed engine
Improved user interface, One-click access for different view modes
Popular "River Of News" view, Grouping headlines by source, date or tag
Possibility to add and remove columns from view
Faster startup and shutdown times, Enclosures and podcasting support
Ultimate configurability, every normal feed can be converted to smartfeed and vice versa
Tagging support, Nested folder support, Full Unicode support, Secure (SSL) connection support
unique algorithm that learns the update frequency of feeds and triggers the updates only when needed