Combine an open source Browser, with Web 2.0 and some Venture Capitalists, throw in the Browser Wars, simmer and pull out.... Flock.
Flock takes a slightly-behind-the-bleeding-edge version of Firefox and integrates into it the idea of a socially interconnected web - your Favourites feed straight from you del.icio.us account and it integrates from the browser with your Blogger (or other preferred) blog spoting site and Flickr photo library (helping along the way to add an extra level of integration to these sites).
Flock is, without a doubt, very neat (by which I mean too funky for it's own good and not quite ready for the prime-time !) and the fact that it's available on Windows, OSX and Linux is a real point in it's favour. I think as long as it doesn't loose direction and really does try to push the integration to become more and more seamless (and hey, I'd love it if they could find a way to support IE rendering as well as Mozilla on Windows if they detect a page needs it) I suspect that unlike Opera they have a clear differentiator in the browser feature wars (at least on traditional computer platforms as opposed to PDA and cellphone devices)