NetNewsWire is a serious RSS newsreader, a feature-filled desktop app specifically designed for voracious news consumers who want more RSS features than Safari and Mail provide. Recent updates leave the interface largely unchanged but notably add the ability to sync with the popular Google Reader service, while eliminating NetNewsWire's old NewsGator service.
This app's three-paned, Mail-like interface presents a shallow learning curve, with an intuitive system for flagging, organizing, and reading news feeds--as simple previews or as full articles with NetNewsWire's integrated browser (or you can set articles to open in your preferred browser). The interface supports tabs, gives you copious options for sorting and managing your feeds (manually or using "smart" settings), and provides navigation that can be as simple as clicking through all your news with the space bar. Not surprising for a power-user app, NetNewsWire also comes with many more advanced features, such as automatic downloading for podcasts (with import to iTunes), flexible "clippings" (to save articles for later, even in different categories), search-engine subscriptions (basically, persistent self-updating searches), and more.
You can pay for an ad-free version, or you can get NetNewsWire for free with a small ad on the lower left side of your interface. Combined with a companion iPhone app, NetNewsWire is an excellent option for news junkies, especially for Google Reader users.