FeedReader is a lightweight, open-source aggregator that supports all major RSS formats. It focuses on the main task--reading and organizing RSS feeds and offering a seamless user experience. The installation file is less than 1.3MB, and, best of all, it's free. FeedReader is licensed under GPL, and its source is available for download, so you can build on it, tailor it to your needs, or even rebrand it for your customers.
Version 2.6 offers extended ATOM feed support.