Opera has been around since 1996-1997 or so, so it's not really very new. It used to be one of the only alternatives to IE. It was ad supported, not sure if it still is. You could pay to get a version without the ads, or use the free one with some banner ads at the top. It's great at web standards, works well with most sites, etc. Nothing spectacular about it, but much, much better than IE. Comes in Mac and PC versions. Not really highly used though, probably the #3 browser out there behind IE and Firefox. It does power a lot of the mobile device browsers though.
Flock is built upon the Firefox architecture. Supposed to be more "social". Eh, I'd just download Firefox and call it good.