Did you know that "Marathon," the classic FPS trilogy from Bungie, creators of "Halo" and "Destiny," has been free and open source since 2005?

We have Aleph One (opens in new tab) to thank for its open source engine, just as id's open sourcing of "Doom" made possible easy-to-modify source ports like "(G)ZDoom" (opens in new tab), Bungie released the source code for the "Marathon" trilogy in 2000 to facilitate the development of Aleph One.

Additionally, Bungie allowed the official Marathon campaign itself to be distributed for free just five years later; those wishing to access Doom's underlying campaign and phenomenal mod community would have to pay for one of the game's many releases.

As for the campaign itself, Marathon was in classic Boomer shooter territory, developing a unique and eerie sense of atmosphere as the series progressed. One can observe the beginnings of Bungie's cryptic, almost mythic storytelling style that would later define "Halo" and "Destiny."

Marathon also has many superficial similarities to Bungie's later work. Green space marines, scoped hand cannons, rogue AI, unobtainable primitive cosmic beings, and other bungie fodder.

Installation is easy; just unzip the files for each game, run the executable, and you're good to go. Within three minutes of my first load of Aleph One's website, I had Marathon Infinity up and running; Marathon, like ZDoom, is a passport to an ecosystem of fan maps and campaigns. Aleph One's website hosts a selection of popular scenarios (opens in new tab), and ModDB (opens in new tab) lists even more.