What game(s) do you love / enjoy that others don't seem to know exists?

https://store.steampowered.com/app/289600/Marauder/

Maybe Centauri Alliance, even though that game does have major issues. It does, however, have some interesting design elements that might be interesting to see in a more modern (and balanced and less buggy) game, like the race that can transforms into various types of creatures, including dragons (despite it being a science fiction setting).

(I didn't find out about it unti long after its release; part of the reason the game is so unknown is that it was released only on two computer systems, and the IBM PC was not one of them.)

Culdcept (PS2), a NA-only port of Japan-only Dreamcast title Culdcept II (with the numeral pronounced as "second", because Japan), comes to mind. Gameplay was a great mix of Monopoly-like "tax passing opponents who land on your territories" board game and Magic the Gathering-like card battles for fighting over said territories. The plot was stereotypical JRPG doggerel. The translation was sub-par, but not so bad as to seriously get in the way of enjoyment (though the printed manual had a couple passages that approached Zero Wing-tier translation quality). All in all, it was a rough gem with insanely addictive gameplay.

Also Ragnarok (DOS), a shareware (now freeware) Rogue-like from the early Nineties -- before the term "Roguelike" had really even coalesced as the name for that type of game -- which featured rare-for-the-subgenre-at-the-time tile-based graphics and mouse-driven interface (though one could still use the keyboard for most everything). There was also a European version called Valhalla that had some differences (I think it had some music, and sound effects for certain things, whereas Ragnarok was silent but for occasional PC speaker sounds), but which I don't think ever got officially released as freeware like Ragnarok did.
This and the Windows-based Castle of the Winds (which it seems a few more people know about) were my first major exposures to what later came to be called Roguelikes (before the term became watered down so much as to be nearly meaningless).

I don't think a lot of people know of Outwars, a surprisingly good 'bug-hunt' game. Gameplay.

Waterboarding.

Harbinger (https://en.wikipedia.org/wiki/Harbinger_%28video_game%29)
A Sci-Fi H'n'S.
I think it's the only Science-Fiction diablo-clone I've come across..Ever.
If anyone know of another one, please let me know!
While being rather short, and the levels a bit bland, the voice-acting and humor goes a long way.

Lost Empire: Immortals (https://en.wikipedia.org/wiki/Lost_Empire#Remake)
A Sci-Fi 4X. (I love science-fiction and space!)
It does have a few drawbacks, being hard to get to work on modern OS's, or even contemporary ones..
It's also not especially original with its research or..anything.
It does have the most amazingly beautiful galaxy though, and the largest I've come across at 5000 stars.

XIXIT

Pathologic 2. You can still find fans of the original who don't know this already came out, its crazy. This is a very good story-based open-world survival game that shouldn't be overlooked. The devs have plans to add 2 more character campaigns, so there's even more to come!

量産型最終兵器ドテチン
(transliterated: Ryōsan-gata Saishū Heiki Dotechin / translated: Mass-production Type Ultimate Weapon Dotechin)

A chaotic freeware horizontally-scrolling shmup with an omnidirectional main shot. Instead of bullet hell, it's more like zako* hell.
*(zako = popcorn enemies; small-fry; literally "minnow")

It's an old game (2007), but it's still available for download from Vector, if anyone's actually curious to try it.

There was a similar thread a while ago and someone recomended a small free game called Perspective.
Short but very enjoyable.

Edit: found the thread: https://www.gog.com/forum/general/the_hidden_gem_thread

Edit 2: Just found out it is on Steam as well https://store.steampowered.com/app/1109410/Perspective/

You are both right and wrong. :)
I own the retail version of the game, so in that sense I know the game.
I just haven't ever installed or played it, so in that sense I don't know the game.

Now that you mention it, I bought it from some bargain bin sale back in the day after hearing some "hidden gem" user review of it.

Anyway, thanks for reminding me of it. I'll play it... some day...

You're only the 2nd person I've ever encountered who know about the game :) From what I remember it's actually kind of great. It has a sort of Starship Troopers vibe going on, and multiplayer deathmatch with those armored flightsuits is a blast! Then again, might be impossible to get it running on a modern system :P

Yeah I wondered about that as IIRC it was an early Windows (9x?) game, and I wouldn't be surprised it has some CD copy protection that at least Windows 10 disagrees with.

Anyway, that is why I have a couple of retro Windows XP/98SE gaming PCs still around. :)