Xbox Live has brought Ultimate Mortal Kombat 3 back to life!
It has all the online modes you would want. It has both player and ranked matches. The ranked ones are straight forward one-on-one versus, and the player matches allow to you chose to fight one-on-one like a ranked game, or in a more tournament style, where two people fight and the other two wait their turn.
What impresses me the most about this game is that most of the games I've played online don't have any lag. Sure there were a couple, but for the most part the games have been really fun. This is, in my opinion, quite an accomplishment and sets the bar for online fighters (or at least 2D fighters, I hope!).
It's not all good, though. Sometimes it's irritating trying to join a game because it'll keep saying the host has ended the session. Also, as I'm sure you've heard, you can only fight at one stage online.
Minor flaws aside, UMK3 lives up to its expectations, gives Xbox Live users a great online fighter, and brings back some good memories of wasting quarters at the arcade.