I recently took part in the Call of Duty 4 - Modern Warfare BETA and it was truly an amazing experience.
To aquire the BETA, you had to register to certain websites and they sent you an activation key which allowed you to download the BETA. They were very rare, they had to be given out daily because the demand was so high.
Well, the gameplay is stunning, the graphics are immense and it truly will be the best game of 2007. Better than Halo, better than Fifa, better than Need For Speed.
Call of Duty 4 - Modern Warfare is the game you have to get, purely for the online experience aswell, Call of Duty started off as a PC game so Activision know how to provide good online experiences.
Yeah, Microsoft made Halo, and they are the biggest computing company out there, but Call Of Duty will put that game to shame.
I hope you enjoy it when it comes out, I pre-ordered it a long time ago :)
All the Best