The best browser for the Mac, Camino, has gotten even better with the release of Camino 1.6. I know what some of you are thinking, 'How can you say that Camino is the best browser for the Mac? It doesn't support extensions, it isn't made by Apple, and it won't buy me a pony!'
All of those things are true (though I'm still hopeful that I'll get a Camino pony in the mail any day now), but for my money no other browser on the Mac feels more like an OS X app than Camino does (your opinion may vary, and I'm sure you'll let me know why I'm wrong in the comments). New in Camino 1.6 are a host of UI changes that make the browser much nicer to look at, as well as improvements in navigating tabs, and a search interface more like Safari's (though Safari's highlighting of search terms within webpages still bests Camino's).
Camino 1.6 is available now, and is free and open source.