The Unity Web player has been around for a while but it is only recently that I have noticed Unity based games popping up on major gaming portals like Kongregate. From what I have seen the capabilities of Unity leave Flash far far behind. Just check out BeGone a beautifully rendered high definition multi-player 3D shooter from NPlay. That is a serious piece of gaming, for free, in your browser.
I am sure those lovely trees and foliage are procedurally rendered but the game is fast and responsive and Unity doesn't suffer from Flash's inability to use the right mouse button. Nplay also do a multiplayer racing game which you can play here. It doesn't seem to be as polished as the shooter but then again I am crap at driving games.