Steam PC games that are absolute classics.

[FaTMaNLeo]

I was wondering what people consider to be classic games, you know the type where you spent hours on and when you're parents told you to get off the console/PC you would argue that it had only been a few minutes. If you're anything like me the first thing that springs to your mind is the old school consoles like N64. However since I don't get as much time to go up to the attic and set up my old consoles I have become dependent on my PC and steam for gaming. So I was wondering what you guys consider the absolute classics on steam ?

Apart from Half Life, Duke Nukem, Doom and Wolfenstein Because they are obviously awesome.