Brent Simmons' OS X-based feed reader, NetNewsWire. It's a fantastic application, and I've definitely got my money's worth out of it. After partnering with NewsGator, I started using their online feed-reader on and off, with mixed results. I like that it keeps my feeds in sync between my computers, and that I can browse articles at lunch, but the interface is still not on par with NetNewsWire itself.
While NewsGator's implementation was lacking, I really did like the idea of dropping the desktop app altogether and going with a fully online solution, so I started exploring other options. The obvious free alternative is Google Reader, and I have to say, I'm impressed. While the presentation isn't as customizable as NetNewsWire, the functionality that I use is all there, and in fact, it has some extra search features that I miss on the desktop.