Bungie's pre-Halo FPS Classic Marathon Infinity just launched on Steam for free, meaning you can now play the entire Marathon trilogy for the low price of nothing
I tried them and in my opinion: trying all three is definitely worth it, to experience their gameplay and see the history. Since it is free, it will not cost you much and if you play as far as you like, you will not lose more than 20 minutes per game.
However I was not good enough to complete the games, so I read the story here and then watched the mandaloregaming videos about the series. And I adore what they did! They pioneered so much in FPS games and the rampancy idea is brilliant, as is Durandal.
This was my first game I purchased with my own money. The atmosphere was freaky and the story was confusing and engaging, filled with unreliable narrators. The puzzles were great fun, too.
I’d love to see it remade into a new engine, but Halo was kind of close, honestly.