Deus Ex is quite possibly the best game ever made. No game is as innovative, well executed, well written, and plain old fun as this. This game never gets old no matter how much you play it. You can play anyway you want and your actions will affect the outcome of the storyline in dramatic ways. If you explore the game deeply it has a lot of cool hidden stuff. It gave birth to the whole good-guy-bad-guy meter a lot of games have today (Knights of the Old Republic, Fable, Elder Scrolls), but this game pwns them all. The beginning of the game drags a bit but if you get past the first 2 levels it picks up. The story line is deep and fun to follow also. What really makes this game shine is the nano-augumentation system on how you can upgrade your-self and really affect how you play. This game is not for everyone, if you want a pick up and play game you will hate this. If you love deep, engrossing games with countless hours of replay, you will not be let down.
Though System Shock 2 had this concept and was around before Deus Ex(one year before I think), it was not executed well and had many flaws.
The name of arguably the greatest game ever created. Within the game 'Deus Ex' refers to the AI Helios, who having knowledge of all available information to mankind and control over every single communication on the globe is like a God, and JC Denton ('JC' debated to stand for 'Jesus Christ'), a christ-like figure who, on the box art looks to the sky as light shines down upon him against the darkness of the dystopian city behind, and who eventually manages to change the world's entire social order at the end of the game.
Tells the story of JC Denton, a technology-augmented government agent who works for an agency called UNATCO, and discovers a hidden conspiracy involving members of the highest levels of government. How you play the game is totally up to you. You can do anything, go (almost) anywhere, and talk to (almost) everyone. The voice work is exceptional, the graphics are great, even years after it's release, and the plot is engrossing. This is definitely a game you should pick up.