Deus Ex, the philosophically charged stealth/action series which set the bar for what a narrative based first-person-shooter should be like!
First published in 2000, the initial game has you play a government agent who gets wrapped up in tons of crazy conspiracies and puts your trust and friendships to the test as you find yourself all over the world trying to find the truth. There are three games in the series: the first, Deus Ex: Invisible War, and Deus Ex: Human Revolution. All follow a similar gameplay formula, and the first and third games are highly revered for excellent atmosphere and level design, exciting gameplay, and a gritty, intricate cyberpunk setting.
All of the games in this series are available for purchase on Steam,, and they are additionally available on PS2, or in the case of Human Revolution, on PS3 or Xbox 360. Regardless, these games are priced like bargain bin deals, and the first game has been widely praised by gamers and critics alike as one of the best games ever made.
If you have never experienced these, I'd highly recommend giving them a shot! You can't go wrong buying a game for 10 dollars. If you're already a fan, here is the topic to discuss anything and everything about these treasures.