The best Xbox 360 games that allow 2 players online on 1 console?

I need a good Xbox 360 game that allows 2 players online.
I'm waiting for gears of war 2 and need a game to keep me busy until then.
I already have army of two, halo 3, and gears of war.
I like shooters and sports games.
Any answers will help.

Thank You
Call of Duty 4 doesn't have 2 player online.
I've rented it and tried.
Doesn't work.
So please don't write Call of Duty 4