- Fact that things taken for granted in most video games have a plausible reason in Portal, even if it isn't explicitly mentioned. Tutorial? You're there to test the gun. Respawning? GlaDos has your brain scanned. Following the plotted line? GlaDos wants you to kill her. It's a Mind Heavy Petting.
- Half-Life. The first one on PC is one of the best first person shooters ever made! It had some of the best levels, best enemies(Stupid headcrabs...), enviroments, music, sound effects (Yes. Those brilliant beeps when you scroll over those brilliant shades of orange are awesome as well.). GORDON FREEMAN! One of the most famous HeroicMimes ever! Oh yeah, he's also a PHYSICIST.
- Team Fortress 2, it's the best multiplayer FPS and especially the best class-based shooter. Each class is so lovingly designed to fit its role. Hell, it's almost like you can play a different kind of game just by changing your class or your loadout. You can sneak around as a spy, build strategic defenses as an engineer, ambush as a pyro, set traps as a demoman, you name it. The development team is awesome at working out bugs and adding new content without destroying the game's balance. Even if the balance does go to hell, Valve is right on top of their game, ready to fix it at a moment's notice. Honestly, I've never seen such good developer-community interaction in my life. Not to mention 2fort2furious is probably the best server I've ever played on, but that's another story. Another cool part about the game is that the developers brought the Team Fortress series back to its class-based roots. The original game was turning into a huge "Stop Having Fun" Guys marathon, with physics glitch-abusing and grenade-spamming becoming the standard.
- Team Fortress Classic. Darker and Edgier, and Fully-automated rotary shotguns - thanks, Heavy.
- While the Left 4 Dead series might not be up there with the likes of Half-Life, Portal, or Team Fortress 2, it's still a very enjoyable series that pays a nice homage/tribute to 80's horror films. It's not scary, but it will make you panic and freak a lot, like when a Tank is coming, or when you hear the moaning of a nearby Witch. And when playing on multiplayer, the experience becomes even more fun.
- Dota 2 is one of Valve's greatest creations, absolute replication of the original game with massively detailed heroes that are very distinct and having unique animations as well as items being unique. You will start with a tutorial that will teach you how to play before you can step into the wonderful world of matchmaking. But the real positive aspect is that it is absolutely free to play, you don't have to pay for heroes, just the cosmetics and they also hosted The International tournament annually, i.e the biggest e-sports tournament of all time. You can get the game for free here.
Gush / Valve Software
Gush about Valve games here!