I'm developing a scenario for the Marathon trilogy, a first-person shooter from the nineties which introduced a lot of gameplay and storytelling tropes that are now regarded as standard in the genre, but never really got much attention because it was Mac-only during the period when it would've garnered the most attention. The series probably has actually gained more attention in the wake of Bungie's creation of Halo than it did during its initial incarnation.
The source code was released shortly before Microsoft acquired Bungie, and the engine has been under constant development ever since. (Notably, an update was released a few years ago that finally renders everything in true 3D). There is an editor for present-day OSes entitled Weland which I have yet to be able to get to run; I have been developing all my content using the old OS9-era editors, mostly Forge and Anvil (the source code to Forge was lost and so the new editors have had to be built from scratch). Unfortunately these have limitations like restricting maps to 1024 polygons (I'm approaching the limit on several of my maps) so I'm going to have to see about getting Weland to work.
I've been developing this scenario off and on for something like fifteen years. The fact that I lost the OS9-running laptop I was building the content on didn't help (I no longer have a functional desktop Mac that will run OS9). I posted an in-development version awhile back but the hosting space it was on disappeared, and I'm going to revise a lot of cringeworthy writing before I repost it. Seriously I was responsible for some truly awful writing that I'm just ashamed of now.
When I've brought it up to a standard where I feel it's not a complete embarrassment I'll post what I've got done. I have no idea if anyone here would even be interested in developing content for an FPS, but just in case you are, I've got a pretty goddamn ambitious one in progress that could probably use content from other talented developers.