underrated or "not so popular" games

• Astrologaster https://store.steampowered.com/app/742520/Astrologaster/
  A theatral game with caustic irony about this past when astrology and medicine were sometimes really close.
  Intermediate english level, i needed an online dictionary to understand it and it was worth it !
  There are some funny short musical interludes !

• Over the Alps https://store.steampowered.com/app/1227400/Over_the_Alps/
  A game where you play a spy and progress through the story stamping postcards.

• Overboard https://store.steampowered.com/app/1546920/Overboard/
  A short game by Inkle : the Heaven's Vault studio. You play a woman on a cruise boat who killed her husband and try to hide the crime before the arrival.

• Blind drive https://store.steampowered.com/app/1300600/Blind_Drive/
  A caustic humor game again where you play a blindfolded driver in a short but crazy story.

Some lesser known puzzle games that I've enjoyed recently:

She Remembered Caterpillars - Great style and atmosphere. The puzzles are pretty abstract and tough so don't be fooled by the visuals; It's not a cute, cozy game!
Toodee and Topdee - I adore everything about this game. The concept, the music, the characters, the puzzles. It even has boss fights! It's a nice mix of logic and action with lots of optional challenges.
Colorful Recolor - It might look like a cheap Sokoban knock-off but the puzzle design here is really tight. I was blown away by how clever the puzzles were. They get devilishly hard by the the end.

Call of Cthulhu®: Dark Corners of the Earth

From the one you already have on steam (and have 0 minutes of play on it) :

Crumble, nice 3d plateforme racing game, 90% positive - 500+ reviews, take around 3-5 hours for main complexion
Sunset Overdrive, a master piece, perfect mix between 3d plateform and shooting games, a must have, 90% positive - 9000+ reviews, take around 10-20 hours for main complexion

Well, Space Pilgrim seems to have very positive feedback so I'm not sure it's exactly underrated, but it sure is quite unknown. I personally enjoyed every part of the series quite a lot. Even bothered to review it which I rarely do since I'm very lazy.
https://store.steampowered.com/app/429470/Space_Pilgrim_Episode_I_Alpha_Centauri/

Obscure and Obscure 2. The first one is really great. The second is pretty bad.

https://store.steampowered.com/app/254460/Obscure/
https://store.steampowered.com/app/254480/Obscure_II_Obscure_The_Aftermath/

Monochroma

Shatterline, real well made f2p fast ttk hero shooter from ukraine. But not many player, which is the biggest issue with it.