Mozilla is perhaps the most versatile and up-to-date web-browser available for linux.
As you mentioned, Opera is also available; however, I'm not a huge fan. I just find that for some pages it is great. Others, not so great.
Phoenix, a branch off of Mozilla, is a nice small web-browser that's main focus is speed. It adds a few things here and there, but the rendering engine shares a great deal in common with Mozilla. If you like Mozilla you'll like Phoenix.
Galeon. Think of galeon as a gnome version of phoenix. It too is designed with speed and efficiency in mind - whilst sharing the mozilla rendering engine.
I'm sure there are many others. There are quite a few I can think of that aren't even worth mentioning.