RSS Bandit is a powerful open source feed reader. Features searches within your feeds, search folders, easy feed detection, excellent synchronization, special folders, and much more. RSS Bandit developers are actually listening to their users and the road map is full of powerful goodies. [Requires: .Net] [Platform: Windows]
Omea Reader is an easy to use, all-in-one RSS reader, NNTP news reader, and web bookmark manager. It's fast, it aggregates, you can organize your information with it, and what really makes it unique are the additional features: lightning-fast searches, flexible filing, contextual access, and extensibility. Comes from the company that develops IntelliJ IDEA, a highly praised advanced Java IDE. [Requires: .Net] [Platform: Windows]
GreatNews "The Intelligent RSS Reader" is an excellent fast reader which features flexible labeling, search channels, synchronization via Bloglines and a clean and usable interface. [Still Free, will become Shareware]
RSSOwl lets you gather, organize, update, and store information from any compliant source in a convenient, easy to use interface, save selected information in various formats for offline viewing and sharing, and much more. It's easy to configure, available in many many languages and the best of all: It's platform-independent. [Platform: Java (Cross Platform)]
SharpReader is an excellent reader without the bells and whistles. [Requires: .Net]
Feedreader is a lightweight open-source aggregator. Feedreader's functionality is focused on the main task - reading and organizing RSS feeds and offering seamless user experience.
Attensa for Outlook works inside Microsoft Outlook to bring up-to-the-minute RSS news and information from your favorite Websites and blogs right into neatly organized Outlook folders. It lets you use a toolbar in Firefox or Microsoft Internet Explorer, to easily add and manage RSS news feeds and subscriptions from Websites and blogs.
Abilon is a small (about 500 kb), fast and free reader. Features actions (You can perform some actions on the information you get such as saving it to a file and running any external program if you want to further process the received data), memories (clippings), archiving and blogging support.
RSSReader is another popular RSS reader.
Vienna is a freeware, open source RSS/Atom reader for the Mac OSX operating system. It provides features comparable to commercial newsreaders but it is both it and the source code are freely available for download.