I've been playing for the better part of the last 27 years, so I lived through many "revolutions" and "revelations" in PC gaming. Half-Life, as mentioned, was one of the most important titles - for reason's a younger gamer won't be able to easily understand. It's not just that it's good as a game. Take a look at older FPS's, and you'll see, for example, a gun floating mid-air in a level, a health-pack / medikit behind a hidden wall... Now, stop a bit and think about all of this: WTF?! I mean, Half-Life was the first game that seemed to understand that, no, a gun shouldn't be floating on a pedestral in the middle of a level, if it wanted to present a realistic story. Thus, it was the first title where, lo and behold, you could find a gun next to a dead guard. Ehrmageeeeed! Realizms in game dezigns!
Before that, Doom was not just a game-changer, but also a system killer. Many youngsters don't even know what "home computers" were, but even those that do, don't realize that Doom was their death. Not old-age, not technical problems, not speed restrictions: Doom. Their processors couldn't spit pseudo-3D graphics like Doom's on screen, and Doom was such a megahit - that created a whole genre, with its clones instantly dominating game sales everywhere - that if you were seriously into gaming, you just had to be able to play them. And on your Home Computer you couldn't. So, you bought a PC. Same goes for consoles, with SNES and Megadrive starting to feel old and restricted when Doom reared its demonic head.
And after that, Quake. I never played it as much as I'd like, but it was a revelation. The first truly-3D environments in a First Person shooter. Followed by Unreal, the first game that brought the action to outdoor environments - there's a reason FPS's where called "corridor shooters" back then! Can you think of a world where each and every FPS takes place inside buildings, and only there? Well, we lived through it - due to technological restrictions.
Those were but a few. Descent was the first action title with truly full 360 degrees movement freedom, Dune 2 was the granddaddy of RTS games as we know them today, X-Wing and Wing Commander the first "cinematic" space sims, Under a Killing Moon and Pandora Directive the only FMV adventures truly worth one's time... I feel bad for youngsters today that seem to play the same things again and again. Gaming seems to have stopped evolving in the last decade, meaning that we haven't seen any mindblowing technological jumps or prominent new genres in gaming itself. The underlying tech gets better and better, but when you've lived through the birth of whole new genres, RTS's, FPS's, third person shooters... Nothing new feels really-really "new". There are exceptions to the rule, but they mostly remain quirky exceptions and not impactfull enough in the long run - like Patapon and Locoroco on the PSP, "one of a kind" experiences you won't find elsewhere, exactly like Doom and Half-Life were when we first met'em.