I don't know if these are necessarily "hidden" gems, but Icewind Dale is a solid CRPG from 2000, there's also a remastered edition if you have a VPN.

https://www.myabandonware.com/game/icewind-dale-db4

https://1337x.to/torrent/2969875/Icewind-Dale-Enhanced-Edition-v2-5-PLAZA/

Also Arcanum: Of Steamworks and Magick Obscura, but I would recommend installing the unofficial patch because it is a Troika game and is such buggy as shit in the original state.

https://www.myabandonware.com/game/arcanum-of-steamworks-magick-obscura-csi

https://terra-arcanum.com/drog/uap.html

goosebumps attack of the mutant

Escape Velocity: Nova - One of the all time classics of open world games being a space trucker that gets wrapped up in galactic events. Sort of unknown because it came out on Mac at first, then was ported much later, then the studio behind it vanished like a soap bubble. The fan community maintains their own set of download links

Star Control II / The Ur-Quan Masters - Open world space RPG that's not quite like anything else. Kind of unfair and old-school (take notes, save often, and rotate those save files), but really a wild story of interlocking open pieces, and fantastic character writing. Not quite abandonware, but the devs open sourced it (The Ur-Quan Masters is the open source name). If you're on Linux, it's probably available in your distro's software center.

Cave Story - Excellent indie story-platformer from just before people started being able to make money off those things. Was originally free, then was given an "upgrade" (made worse) by a company that royally screwed over the original dev. The fan community maintains their own set of download links including a modern engine rewrite that works on modern systems. (If you want to actually pay for this, don't, and instead buy Kero Blaster)

dreamweb is really cool if you like point and click adventures.

Cybermage: Darklight Awakening - FPS, Sci-fi, Super person saves the world from an alien invasion gone wrong. Some overlap in publisher/developer with System Shock. (You're gonna need to figure out how to jump because its not listed on the in game key listing and I've forgotten what combination of keys it took to do it.) Pretty solid game after you get the hang of the "magic powers" system.

Heroes of Might and Magic. The earliest installments are abandonware ... or at least findable on abandonware sites. Solidly enjoyable for a bit before it gets monotonous but I just use that as the sign that its time to stop playing and go do something else.

Transformers: Devastation is ... fuck... its amazing. There's a few versions uploaded to the Clearance Bin section at Archive.org. Watch some YouTube videos to see what you're getting in to. Pick one of a handful of the Generation 1 Transformers and brawl your way through 3D levels that and pretty much "play" an episode of Transformers. Characters level up, there's some weapon crafting, some looter shooter mechanics, some hard to find pickups that unlock character/scene art (unlocked a picture that I had on a lunchbox when I was in elementary school that I'd totally forgotten about). If you ever wanted to fist fight Devastator with Bumble Bee or pile drive a Stuntacon with Grimlock, this game will not disappoint. I was really surprised how well this ran on my toaster of a computer.

Clive Barker's: Undying. FPS with some Lovecraftian Horror themes. A bit heavy on the jump scares but there's a few parts that I found genuinely creepy. Which they didn't give to the hero "jokes" to say, really gets in the way of the creepy horror vibe. There's some CD keys in the comments, one of them worked for me.

Space Crusade. Warhammer 40k turn based squad combat. All the way from 1993 to fulfill your "I rolled 4 dice + a reroll and all I got was five zeroes" needs.

Edit: AAaaaaannnndddd I just checked and it looks like Planescape: Torment is available on abandonware sites. Maybe I'll get around to trying to beat it.

Cortex Command

Beneath A Steel Sky was abandonware for years and years, and is now technically just freeware. it's so free that it's just straight up downloadable on steam. It's a very strange sci-fi adventure game with a tone problem that makes it one of my favorite games ever. It's not the best adventure game, but it doesn't have too much "moon logic" for its puzzles, and the art direction is so grimy in a way that's really lovely and kind of unique compared to most dystopian fiction. I feel like it'd be best to just point you to it to go experience it than detail it too much, but you are a guy forcibly brought in to a dystopian city after your village is attacked, and the rest of the game is a kind of descent into this highly industrialized, capitalistic fascist dystopia. It's neat.

It has a sequel called Beyond A Steel Sky. It's pretty solid, but it's also, imo, a completely different game. There is exactly one level that evokes the aesthetics and sensibilities of the original, and it's got these "HEY REMEMBER THE FIRST GAME?" bits bolted into the story, but you could excise all of it and lose nothing, so it's hard to recommend it as a followup-in-spirit, but it's a solid sci-fi adventure game with a lot of modern amenities (read: no soft locks, no impossibly obtuse puzzles, regular autosaving), and it has interesting stuff to say along the way so vOv I'd recommend it, even though it isn't necessarily abandonware

No One Lives Forever

First entry is a little clunky but good enough to warrant a cult classic sequel. Then there was Contract: J.A.C.K. which stripped away most of the novelty of NOLF 1&2 and the series kinda fell off after that

https://www.myabandonware.com/game/nocturne-bgz

Nocturne is good. Also freelancer. Frogger is good fun https://www.myabandonware.com/game/frogger-9zn

There's actually loads but I'll need to boot up my win xp laptop to remember what I installed.

A lot of the old battlefield games can be played with bots from 2142 and before.

Unreal Tournament '99 and '04 are abandoned, and available on Internet Archive ['99] ['04].

The neverhood is very nostalgic for me and i think on abandonware sites

claymation point and click from the 1990s

no recs, but I'm noticing that website could use a lot of cleanup regarding game genres.