GOG has a lot of great classic games available, but for a bit of fun, I want to know what some of your favourites are. Bonus points if they are more unknown, obscure or niche.

They must be available on the GOG store (I might want to buy them) and a reason why you like them would be helpful.

Slain - heavy metal platformer and pixel art done right.

The Real Texas. A strange, surrealist adventure/RPG that apparently forgotten by almost everybody.

Good, rather obscure game on GOG:

IGI-2: it's like a poor man's Deus Ex. Large unlinear levels some with huge height differences, awesome sniper rifle, but somewhat let down by wonky AI; enemies have super human hearing but are blind as bats.

Evil Islands: CRPG with open world to explore and best crafting system I've ever seen. Interesting and novel character/skill system, and quite hard (it's Russian).
Shadows and camera don't work properly on GOG version, though, something which really should have been fixed years ago (you can fix it easily yourself).

https://www.gog.com/game/anachronox

A hilariously funny Western JRPG (yes, you read that right) with fun and inventive mechanics.

(Was supposed to be the first part of a trilogy. Someday..maybe. Hopefully)
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https://www.gog.com/game/arcanum_of_steamworks_and_magick_obscura

A deep WRPG where magick and technoloy exist side by side.

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https://www.gog.com/game/imperium_galactica_ii_alliances

A RTWP Space 4X with many good mechanics that still hasn't been copied.

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https://www.gog.com/game/space_empires_iv_deluxe

Another Space 4x. A Turn-based one this time.

(There is a V, but it made a few changes that not everybody liked)
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https://www.gog.com/game/beyond_good_and_evil

A wholesome adventure game with a riveting tale and many fun side-games.

(A sequel is in the works, has been for quite a while. Doesn't look like it'll play the same at all)
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https://www.gog.com/game/outcast

Another Adventure game. Early voxel-based graphics.

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https://www.gog.com/game/giants_citizen_kabuto

A third person shooter with humor. Base-building in the excellent LAN/MP mode.

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https://www.gog.com/game/sacred_gold

A forgotten H'n'S game. Big, open world. Ride a horse.

(Don't play Sacred 3. There is no 3!)
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https://www.gog.com/game/drox_operative

Uh. You pilot a spaceship, but it's not a STACS game (Spaceship Trading and Combat Simulator). It's more like a H'n'S game in the gameplay loop. Oh, and it is set in a universe where the NPC's play a 4X game.

It's good. DO2 is in the works too (and on GOG).

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https://www.gog.com/game/conquest_frontier_wars

An RTS where you do things a little differently.
It's almost like a 4X game, but with only extermination.

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https://www.gog.com/game/tachyon_the_fringe

A Mission-based cockpit-only Spaceship sim, like Descent or Starlancer.
Main character voiced by Bruce Campbell of Evil Dead fame.

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https://www.gog.com/game/impossible_creatures

Another RTS with interesting and unique mechanics.
Collect animal DNA in campaign mode to overcome your current foe.
Use 2 of any and all animals in MP/sandbox mode to create the most effective army.

TRI: Of friendship and magic - https://www.gog.com/game/tri

The Last Federation, an ambitious indie strategy game/RPG where you play as an alien demigod manipulating different species, each of which has its own rules and gameplay mechanics. The combat is turn-based bullet-hell, only time I've seen such a thing. You can really tell they went all-out to make something new and fresh, but you can also tell there was no way in hell this could be properly balanced, not with all the asymmetrical mechanics at play. Still worth a try if you find the premise interesting.

Avernum is about as Niche as it gets. So picture this. A single guy with little to no assistance has been making games since about 1995.

Now here's today. He's remade the games twice, and people still find reasons to complain about the zoom level or something like he can make the pixels physically bigger.

E.Y.E: Divine Cybermancy 

A weird Deus Ex like FPS set in a Blade Runner like cyberpunk city. Kinda hard to explain and is far better to just be played.

https://store.steampowered.com/app/91700/EYE_Divine_Cybermancy/

Eador - not sure how unknown this is to the majority of gamers but it was pretty obscure to me. It is detailed and complex if you're into turn based strategy games. Mix of HOMM and Kings Bounty in terms of castle building but with a lot more options. There's a campaign and random world generation also. Four different hero types, quests, resources, so many troops, good vs evil element, and more. Any fan of this genre and hasn't played it would really like it I think.

Z.A.R. (Zone of Artificial Resources)

https://www.mobygames.com/game/zar

https://www.mobygames.com/game/zar-mission-pack

An old Russian FPS with destructible terrain. I really liked it. It's on Steam (surprisingly) but the mission pack which came out later is nowhere to be found besides in Russian. English version used to be on ISOZone, but since its closure, all I could find are russian sites where I guess it could be downloaded, but I have 0 comprehension of russian language so...

Made by Maddox, now owned by 1C Company, one has to wonder why it's not on GOG as they have tons of games here. One even directly from 1C: Maddox Games (IL-2 Sturmovik™: 1946).

EDIT:
Actually, I've just sent an e-mail to 1C, asking them about releasing ZAR + the mission pack here. Hopefully, something comes out of it.

There are some cool looking games to check out, many I've never heard of. Awesome.