Coke or Pepsi? Kirk or Picard? Betty or Veronica? The great battles of the marketplace tend to be duels, and few people gripe if you leave out RC, Sisko, or Cheryl Blossom. The "Browser Wars" are no different, with "IE vs. Firefox" having replaced "Netscape vs. IE" long ago, and other options are often forgotten. Opera has been one of the strongest alternate browsers for a long time, and it was my browser of choice prior to Firefox. Opera 11 (free) continues the Opera tradition of doing something different instead of a minor reskin of someone else's codebase, and delivers a plethora of features that are actually designed to be usable, not to pad out a checklist.
Opera first won me over with tabbed browsing, and it took a while before its competitors leapfrogged it. Opera 11 still has tabbed browsing, and it kicks it up to...uh... 11. While Firefox needs add-ons to get the tabs on the side (a boon for monitors with more width than height), and Chrome can't do it at all, sidebar tabs are built in to Opera 11. No matter where you like your tabs, though, you can slide one into another to easily group them together. If you have them on the side, you also have the option of large thumbnails, handy for navigation (and you can turn them off if you need more space).
Indeed, "You have the option to..." or "If you want..." seem to be the guiding words for Opera. In only a short while, I found a lot of ways to configure my setup to browse just the way I like. There are many toolbars, panels, and features which can be activated, deactivated, moved, or customized, until you're completely comfortable with your environment.
You can choose to tile or cascade tabbed windows in Opera. (A minor annoyance: You can't "revert all to tabbed," though you can maximize each one as needed.). Perhaps more important, you can create a new tab as a "Private" tab, and any tabs created from that will also be private. Private tabs do not save their history, cookies, or caches. This is much more convenient than Firefox's Boolean browsing, in which you're either in private mode or you're not; mixing both types in the same session adds a lot of flexibility.
Another unique feature of Opera 11 is Unite. Unite allows anyone to trivially set up file sharing, streaming media, and so on, all from within their Opera browser. These features can be password protected for privacy or made public. Obviously, the bandwidth of most users will not sustain massive amounts of such activity, so these features are best when used for small groups of friends or coworkers.
The only real drawbacks to Opera 11 that I was able to find were that some sites -- such as Netflix Streaming -- do not support it, and there are some add-in tools I've come to rely on in Firefox which do not have equivalents in Opera 11. Also, it doesn't find your other browser's bookmarks and offer to auto-import them, though you can do so manually. Beyond that, there's nothing which instantly stood out as a reason not to use Opera 11, and I found the speed and customizability to be sufficient reason to keep it as my primary browser for now.