SharpReader is a RSS/Atom reader that can handle most of the popular formats and versions.
Key features of SharpReader:
Handles all RSS versions, ATOM 0.3 and 1.0, modules like dublin core, content:encoding, xhtml:body, etc.
Advanced threading support allowing you to view connected items together in a threaded fashion. SharpReader detects and shows connections between items if they have same link, if one item links to another, if two items both link to the same external webpage, or if an item has comments (for feeds supporting the standard).
Group subscribed feeds into custom categories.
Feed settings like refresh-rate and purge timeout can be set per feed or per category. Category-wide settings apply to all feeds in that category that are still set to "Default" for the setting in question.
Dialog-less way of subscribing to new feeds - just drag a link from your browser into SharpReader, or enter the url into the address-bar at the top.
Feedster integration to easily search weblogs and newssites for specific terms, and even subscribe to such a search to be notified of new results.
Support for proxy-servers and proxy authentication.
Reduces bandwidth by using HTTP Conditional GETs and gzip/deflate encoding.
Minimizes to the system-tray.
Systray popup when new items arrive (can be disabled on a per-feed or per-category basis through the properties pane).
Easy keyboard navigation to go the next or previous unread item.