Old, grizzled veterans of the browser wars (Blood! Honor! W3C Standards!) will remember OmniWeb, an expensive, Mac-only überbrowser that always seemed to be slightly ahead of everyone else. Well, it's free now.
A little background: OmniWeb is based on a slightly modified version of WebKit, but used to differentiate itself from other browsers with novel features. Ad blocking, tab previews, saved browsing sessions, inbuilt RSS reading, single-windows browsing and per-site preferences could once justify OmniWeb's price, but now most of these features, and plenty more, can be found in free browsers. That's not to say that OmniWeb isn't a good browser, but it'll have a extremely hard time facing up against our increasingly capable choices in free software—especially the fantastic Safari 4.