chimera
I just started using Chimera as my primary browser on OS X. It's a lightweight, Cocoa-based (i.e. native UI, non XUL) browser based on the Mozilla "Gecko" rendering engine. It's still pretty young, and some of the features in Mozilla I use heavily aren't there yet (bookmark keywords, mainly, plus the Movable Type bookmarklet doesn't work). On the other hand, it's MUCH faster and uses considerably less RAM than Mozilla or IE. I like it a lot so far.