At last night's PlayStation Showcase, the Destiny developer Bungie revealed that it is reviving one of its earliest sci-fi titles Marathon as a "sci-fi PvP extraction shooter", and that this new game will be coming to PS5, Xbox Series X|S, and PC.
However, if you like your games a little bit more old-school (and let's face it, you're here, so you probably do!), then it's worth pointing out that you can grab the original Marathon Trilogy (Marathon (1994), Marathon 2: Durandal (1995), and Marathon Infinity (1996)) online for free, thanks to the enhanced open-source engine Aleph One (thanks Wario64 for the spot!).
To give a little background, Bungie basically released the source code for Marathon 2 in the year 2000, which enabled the development of Aleph One to begin. Then, in 2005, the company followed the release of this source code by publishing the trilogy's game content for free online without a license, allowing people to openly redistribute them, so long as it was for free and not-for-profit.
In 2011, the Aleph One team, therefore, decided to start redistributing the three games packaged together with Aleph One's binaries for no cost at all, creating an easily playable and enhanced version of the classic shooters. Interestingly, Bungie actually seems to have supported the project, writing about its announcement on its website in the past, and even granting the group a limited license in 2021 to redistribute Marathon's content. In other words, it seems like there's no chance of this being taken down anytime soon.
You can find the download links you require over on Aleph One's website. Enjoy!