Did you know that Halo and Destiny creator Bungie's classic FPS trilogy, Marathon, has been free and open source since 2005? I sure didn't when I shelled out cash for them on the Xbox Live Arcade in 2008 like some kind of chump!
We have the Aleph One (opens in new tab) open source engine to thank for Marathon with no money down. Similar to how id's openness with Doom allowed modding-friendly source ports like (G)ZDoom (opens in new tab), Bungie released the source code for the Marathon Trilogy back in 2000, facilitating the creation of Aleph One.
In an added treat, Bungie allowed free distribution of the official Marathon campaigns themselves just five years later. If you want to access Doom's foundational campaigns or phenomenal modding community, you're still going to need a paid copy of one of the game's many releases.
In terms of the campaigns themselves, Marathon is in classic, boomer shooter territory, and the series developed a unique, eerie sense of atmosphere as it went on. You can observe the beginnings of Bungie's cryptic, almost mythic style of storytelling that would later define Halo and Destiny.
Marathon also has plenty of surface-level similarities with Bungie's later works as well. You've got your green space marines, scoped hand cannons, rogue AIs, and unknowable primeval cosmic entities—Bungie's bread and butter.
Installation is a breeze: just unzip the file for each respective game, run the executable, and you're good to go. I had Marathon Infinity up and running less than three minutes after I first loaded the Aleph One website. Marathon's also a passport to its own ecosystem of fan maps and campaigns, just like ZDoom. The Aleph One website hosts a selection of popular scenarios (opens in new tab), and even more can be found on ModDB (opens in new tab).