The goofy run-and-gun FPS Serious Sam: The First Encounter was released in 2001, setting Croatian developer Croteam off on a trajectory that would eventually lead it to talkative philosophy puzzler The Talos Principle. (I'm not sure how that happened either.) It was really good, and spawned a number of sequels that followed no particular numerical or chronological order: The First Encounter was followed by The Second Encounter, after which came Serious Sam 2, then HD versions of the Encounter games, but not Serious Sam 2, because it kind of sucked.
A fan-made update of the first two games called Serious Sam Classics: Revolution launched on Steam Early Access more than five years ago, but the project went dormant near the end of 2016; development team Alligator Pit said that the long silence was due to "life happening." But it was recently brought back to life by Croteam, which took over development and helped get this project through the door.
Serious Sam Classics: Revolution includes the full First and Second Encounters, along with new enemies and a new boss, eight new campaign maps, a new Survival mode and new multiplayer modes, and the full Serious Editor and Serious Modeler. The old Serious Engine has been updated to be compatible on 64-bit operating systems, with "advanced graphics shader support" and Steamworks integration. Achievements, cloud saves, leaderboards, controller support, and other updated Steam features are also supported.
This is, to be clear, an update of the original releases and not the HD remakes, although the general consensus among Steam reviews is that it's a similar sort of overhaul, but with some newer effects and better performance. It's also quite a bit cheaper: The HD First Encounter normally goes for $15 and the HD Second Encounter is $20, compared to $10 for Classics: Revolution on Steam. Owners of the "classic" Serious Sam games on Steam get it for free.
In case you'd forgotten, Croteam is also currently working on a new Serious Sam game, Planet Badass. We had hopes that it would be out this year, but a release date hasn't yet been announced.
