ExtremeTech has an excellent review of the three new models of browsers this year from Mozilla, Microsoft and Opera:
"Right at this moment, big changes have or are about to occur in three well-known browsers: Internet Explorer is finally being updated, with version 7 in its third beta and almost ready to roll out the door; Firefox is also ripening an upgrade beta for its Version 2.0—it's in beta 1; and finally Opera, which has a devoted but smaller following, has recently come out with Version 9.0."
Check out the feature chart for an overview. In terms of web standards compliance, Opera is the best so far - they also get praise for tab implementation. IE7 has implemented much of the functionality that was already in the other two browsers (tabs, extensions, etc), while Firefox has its large collection of extensions and programmer hooks to recommend it.
Overall though, ExtremeTech has no preference between the 3 offerings:
"These are all fine apps; we recommend you give them each a spin—it's a free download, after all—and stick with the one you find the most comfortable."
It's still early days, but one thing is for sure - all this competition is promoting a lot of innovation!
Incidentally, Opera just today announced a new platform for widgets - called Widgetize!. This service allows users to create their own desktop widgets.