Injustice Gods Among Us - Flash version of PlayStation Batman Game
Controls are 'A' and 'D' to punch and kick, 'S' to block enemy attack and arrow keys to move.
In the original game we have Superman gone bad, because of Joker who got him drugged and made him kill his own wife. After the nuclear explosion, a parallel Universe opens up and sucks in most of major Justice League members, turning them into The Injustice League. Bad Superman is the rules of the Injustice Universe, while the Batman is the leader of the rebels who wants to take him down. The point is that good Superman now must team up with the Batman and his rebels to bring things back to normal. Anyway, this game has nothing from this plot, and it's just nothing more than a fighting arcade, it contains only fighting scenes, where you can pick your favorite characters. If you'd like to read it, the original game's review is here: Injustice The Gods Among Us full review
I'd say that despite good fighting scenes, the game feels a little bit unpolished. For instance, it won't let you cancel the fight and go back to main menu if you want to. There's a back button only in the beginning of the fight and later it disappears. The only to way to achieve this is to refresh the whole game page and start all over. Another thing I didn't like were fights against yourself. Those fights are usually really messy, you never know who's who and which side you're on and who's your guy at the moment. That should have been cut out of the game.
Below, you can see some of the opponents: Killer Frost against the Bat-girl,

and Superman vs Green Lantern

And what about Batman? Where the hell is he? Well, as the leader of the rebels, he must be somewhere in the near, probably sharpening his batarangs. Joking aside, he's definitely watching the match because that's the Batman's voice you hear announcing each round.
Let's Play Injustice Gods Among Us (Flash Game) - a YouTube video (walkthrough)