If your 'client' can handle a zombie horror shooter, then yes, Left for Dead is awesome. LfD2 is also pretty awesome, but slightly less awesome than the first one.

Are you looking for a 'M'ature shooter, or a standard 'T'een shooter? All the classics (CoD, Modern Warfare) are T.

In the M category you will also find Bulletstorm: it's more fun and less realistic... it's over the top. Play is enhanced with beer.
Also, on June 14 will come the release of the most anticipated shooter of the decade--because it's literally taken ten years to produce it--Duke Nukem Forever. It's very very 'M'.