The Marathon trilogy. Before Halo, Bungie did another FPS called Marathon, and its sequel Marathon 2: Durandal. Marathon 2 was the game I picked up at the same time I bought my very first Macintosh. Marathon Infinity came out not too long afterward, designed by a sub-developer and doesn't carry the story from second to third all that well. But still, it was a great game series for the '90s. Bungie made the game engine open-source, so people could still play their old games on modern operating systems and hardware, and later they made all the game files open-source too. So now you don't need the original CD-ROMs either. And they're all available for download from the Aleph One website. There's also loads of game mods, too. Last year I ended up downloading and playing all the way through each game, then churning through several mods like Tempus Irae. I still fire up the final level from Marathon 2 from time to time, just for the hell of it.