Don't know what qualifies as "unknown" to you, but do you know "Fantasy Wars"? If not, you should!

How about some rare games in general? :P They may be difficult to find, but some great gems are Fire Fight and Urban Assault! Fire Fight was a unique Win95 shooter game, and Urban Assault was an innovative futuristic RTS!

Defender's Quest

Have you played Bad Mojo? It should be right up your alley:

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

2nds for Project Eden and Geneforge Saga
also:

Pathologic
Inquisitor (wink)
Rex Nebular and the Cosmic Gender Bender
Spellforce series
Shadow Man
Spycraft: The Great Game

DOOM... Oh wait.

Because you're a crazy guy who kills friends! :P

Jazzpunk.

Great game, particularly the first half, with lots of lovely retro humour in it.

Pax Imperia 2, an almost forgotten 4x gem. Luckily GOG found it and brought it here.

HeXen and Heretic!!!!!

Snakebird. A fantastic puzzle game with clever levels and adorable art.

Makes me sad that so few people seem to have played it.

"Evil Islands" still has the best crafting system i have ever seen in any game. So much freedom. There are 2 steps. First you may create an item from various recipies. Lets say you can create dagger from stone or bronze or adamentium. All these elements have basic energy. This is the simple part. And then you can add "ANY" spell to these items. You may add "weakness" spell to that dagger so it weakens the enemy on successfull hit. Or you can add fireball. Or you can put "healing" spell on your armor so everytime you are injured it auto heals you.

Ah and ofcourse spells, you can put together runes and create bigger, wider, longer spells. Like forexample you can make your "weakness" spell to last for 6 seconds or 12 seconds. With every rune the complexity of the spell increases and you need to have enough spell level to be able to cast that spell. Or even to create it. There is infinite possibility.

Problem is its neither on gog nor on steam and i couldnt find it anywhere so i had to pirate it. Good luck.

AquaNox 1 and 2 are fantastic shooters... if you like decent and wish it was under water then this is for you. I am a spacesim notjob and i love these and hold them in the highest regard.

Blood 2 IMO is one of the masterpieces of the late 90's it continues to be one of my favorite 3D shooters of all time. Great level design, fun puzzles, good shooting mechanics, awesome weapons (cerebral bore anyone?)

The Chronicles of Riddick: Assault on Dark Athena and Escape from Butcher Bay are bar-none the greatest games i have ever played. And i feel the entire experience is beyond the batman games in story and stealth/fighting mechanics. its as violent as you want them to be :)

Duke Nukem: Manhattan project is a side scrolling GEM, it has all the duke you could ever need and that early 90's nostalgic side scroller awesome feeling you crave. They got everything right with this... and the music is just perfect!

Giants: Citizen Kabuto is one of the funniest and most diverse games you will come to see. It will remain a constant level of comparison for anyone trying to make a "funny yet fulfilling" game. I would say get this over any Double Fine game and see what a true masterpiece is :)

Jade Empire was the first game i ever fully completed in my life. it started me on my gaming journey to really look at how a game is made and appreciate the art/story/flow. I have this on the xbox (bought just for this) in retail and now on GOG if that says anything.

Dont know if Painkiller is an unknown game but its one of the best run and guns ever designed, tops serious sam in my book.

Slavezero you are an pissed off robot with a ton of massive weapons in a small world. its a corridor shooter in the vein of mechwarrior (city maps) with a decent story (if you dive to it) and a great "fight to survive" feeling. the music, for the time, was some of the best more adrenaline pumping tracks i have ever heard since TrickStyle, which is the best future "tony hawk" skate simulator that exists... think Tony Hawk meets F-Zero ^_^

Sanitarium!!!!! the only adventure game i ever played beat, played again, and then 3 more times. I hate adventure games but this keeps me coming back. Its fully voice acted in all levels, the story is mindblowing and worth the time and effort to sink into it.. and other then the very first big puzzle its pretty easy to navigate. but not to easy... the length is perfect and the music is so so so amazing. I cannot praise the music, sounds and atmosphere enough. This should be purchase #1 for all gamers on earth.

SHOGO harkens back to the glorydays of monolith. like Blood 2 this was an ill received yet amazing game. Its a combination of Slavezero as your a mech pilot that just owns everything and blood 2 a foot soldier in a future-imperfect world. the story and pacing are spot on and again the music really sets the game.

Two worlds 1 and 2 were laucnhed when other bigger open world games were released... the voice acting is horrible and the animations are cring worthy but... something about it makes you keep playing :)

Unreal 2 was one of those games where i didnt want it to end... minus a roof top "tower defense" level the rest of the game was fantastic. It was the very first Mass Effect if thats possible. do a mission then return to hub collect story pieces and then go on mission. I cried at the end, really... i got emotionally attached to this game. Then again im a massive unreal fanboy so there is that :p

all these are available on gog.com

Dangerous High School Girls in Trouble!

It might be a casual game, but it's also very unique and immersive. And it tells an interesting story. You have to read a lot though and you should definitely have a good knowledge of the English language.