If you don't have it already buy The Orange Box, it's a great deal and features a pretty terrific single player experience, a well conceived puzzler and an stylish class-based multiplayer shooter. Can't go wrong with it really.
Otherwise you might enjoy Bioshock, I got really hooked on it but it has a lot of flaws, mainly concerning it's console-itis and some other stuff still it's a pretty fun game. Be sure to get patch v1.1 which definitely fixes a lot of issues (like the vita-chambers, giving you an option to disable them or the mouse acceleration plus it adds the missing xbox360 tonic and adds a few plasmids as well).
That being said, you already own one of the best games of the year and that's Call of Duty 4, the single player is pretty intense (even if short) and multiplayer is loads of fun.