Not so much free, but more of a “freemium” application, but NetNewsWire has become an essential part of my Mac and iOS life.
For iOS, it’s free and paid, where the free version has ads. Same for Mac OS X. While I don’t like the ad-supported stuff, I appreciate that they give it away for free.
But what is NetNewsWire? It is an RSS reader that syncs with Google Reader, but that is essentially all it does, which is unfortunately the one thing that makes it not very good for very many folks.