This game probably changed my life. It was one of the first video games I've ever played (I played when I was around 4, my cousins brought a copy of the game and put it on our xbox), which opened a whole new world to me, of what a video game could be. I was familiar with racing cars endlessly without story, Minecraft which was infinite, and other stupid iPhone games that I had. None of which had a real story, or objective except "get through it". This game completely flipped that around, having a story mode with lots of chambers. I'm not gonna spoil it, since it's one of the best stories in a video game I've ever played, but basically, you are woken up in a facility, you meet this crazy blue sphere and accidentally wake up a killer robot who tries to make you test and... well... just see for yourself. But really, this game has so much content and so many hours of fun to be had. The steam version lets you download chambers from the internet (SOME OF WHICH ARE INCREDIBLE), the soundtrack is INCREDIBLE (especially the ending Jonathan Coulton song, but I will admit I am a sucker for anything JoCo), and after you finish campaign, there's a co-op mode. Valve really outdid themselves with it though, it has a completely different story, new levels, new characters and mechanics compared to the campaign. Do play it. This has to be my favorite game of all time, thank you Valve.