Please don't say the word 'Opera' in context with Macs. Please! IE, Opera, and iCab are essentially worthless for Macs, from bad to worst in that order. It was valiant of Opera to try so late in the OS X browsing game, but it just doesn't cut it. IE was cutting edge when it was virtually the only option for OS X, and iCab is basically an OS 9 port. 'Nuff said there. My personal favorites are Camino and Shiira. Camino has the Gecko (Firefox/Mozilla/Netscape) rendering engine, with a beautiful cocoa interface. It's got some bugs but I love it. Shiira uses webkit like Safari, is faster than Safari, and has some other small features that I like (such as a downloads/bookmarks drawer and re-orderable tabs). Both Camino and Shiira have very nice UI in general. Additionally, Shiira has some cool options standard in its preferences, and ones you can get with Camino ExtraPrefs that enhance browsing experience a good deal. (Namely, pipelining enabled and agent spoofing.) Firefox is nice, but I like the way one developer at MoFo put it: "Whenever I'm using Firefox for OS X, I get the feeling I'm using a program that's pretending really hard to be a native Mac app."
Edit: Also, if you're looking for a goofy little browsing option that has some novel features, go to Sunrisebrowser.com and give that one a shot. It's got the ability to use transparent windows, and some other HTML source stuff that's kinda weird but neat.