Spec Ops: The Line is free on Humble for Steam and runs on PC, Mac and Linux.
"Spec Ops: The Line is a new original title from 2K Games that features provocative and gripping Third-Person modern military Shooter gameplay designed to challenge players' morality by putting them in the middle of unspeakable situations where unimaginable choices...
This was such a good game. Very dark. Most compelling shooter I've played so far.