My personal favorite right now is Mozilla or Camino
Camino looks nicer and is leaner, mozilla has mail built in with a really nice junk filter!
Noteable features are popup blocking, and tabbed browsing (apple click on those links)
You can also block banner ads individually by Control-clicking on a banner and selecting "Block images from this server" (only in mozilla right now)
Safari Apple's first attempt at a browser, and all I have to say is they did a pretty good job! It blocks pop-up ads, and will soon have tabbed browsing. Only OS X though.
iCab is worthwhile, it will even block banner ads as well as pop-ups. Has better support for pre OS X than Mozilla.
Opera has been around for a long time, very lean and quick.
Omni Web had the first OS X browser.
Netscape is a lot like Mozilla, because is is mozilla! But they have added quite a few of their own features, and is a much bigger download.
Internet Explorer Most likely came with your mac, and most web pages are designed to work on it. It won't block pop-up ads, it does not do tabbed browsing, and is not a zippy when compared to any of the others on the list.