Macs come pre-packaged with Safari, making it your default browser. In itself it is a pretty good application. Safari loads quickly, supports tabs and pop-up blocking and is compatible with most web pages. On PC, many users have left IE for the safer and more efficient Firefox. The Mozilla browser has yet to become a reference on the Mac though. Even though you get all the add-ons and extensions, I still think it's slow and eats up way too much memory. Multiple bugs have surfaced in the Mozilla browser on Mac, like when it suddenly locks up for no reason, several times in a day.
Luckily, there's plenty of great alternative browsers on Mac. Opera loads up very quickly and is surely the safest browser out there. Although the interface is somewhat cluttered, if you take some time fiddling with it, you can customize it to your own liking. It also handles OSX like widgets and can open up new windows in multiple different ways.
Although Omniweb is shareware, this is the browser for productivity nuts and business types. Apart from creating your own workspace, it also saves your sessions and can build shortcuts to your favorite sites. Very functional indeed.
Shiira looks and feels great to use. Try the thumbnail view to get a quick glance of all your web pages open. It also has page transition and sounds effects and features private browsing.
I may personally not be that fond of Flock, but I can understand if other people are. Blogging freaks will like how it facilitates blog posting, via a simple WYSIWYG blog editor, saves favorites and can upload your photos to your Flickr account.
Camino is my personal favorite. It loads up very quickly, works hand in hand with the Mac Keychain and has a very fresh and clean look. It's easy to import and export bookmarks and now has spell checking and can restore sessions. To me, it's like a more efficient version of Firefox, albeit without add-ons.