https://store.steampowered.com/app/339120/Fork_Parkers_Holiday_Profit_Hike/
pretty good game

Classics!!!
https://store.steampowered.com/app/380840/Teeworlds/
https://store.steampowered.com/app/11020/TrackMania_Nations_Forever/

Maybe not the best, but I like it. :)
https://store.steampowered.com/app/260410/Get_Off_My_Lawn/

A personal favorite of mine Cinderella Phenomenon

Also, added all yours to my wishlist

Some of the ones I've played are:
https://store.steampowered.com/app/605610/Dawn/
https://store.steampowered.com/app/648390/Maries_Room/
https://store.steampowered.com/app/532840/CAYNE/
https://store.steampowered.com/app/669630/Hollowed/

Puzzle game :)
https://store.steampowered.com/app/672730/Find_You/

https://store.steampowered.com/app/439940/Lucid9_Inciting_Incident/

For those who are into visual novels. Made with literally no budget, and a free game. Second part coming, but no dates, although they are working on it.

It's pretty linear game though, some bad end and a true end, plot based story, no heroines routes. yet

The Game isnt of the highest quality but its Really fun.
https://store.steampowered.com/app/700330/SCP_Secret_Laboratory/

i have started this one yesterday its really fun (and casual) Gems of War - Puzzle RPG

also this one is superb (for those who like quest games like KQ) Heroine's Quest: The Herald of Ragnarok

Super Crate Box
Pretty fun arcade game.

Thanks for all your suggestions. all added to the list. ^^

https://store.steampowered.com/app/409160/Dr_Langeskov_The_Tiger_and_The_Terribly_Cursed_Emerald_A_Whirlwind_Heist/
https://store.steampowered.com/app/467360/OffPeak/
https://store.steampowered.com/app/266430/Anarchy_Arcade/

Evening Surprise
About 20 min long visual novel. Just play it sometimes when you need something to do for 20min, and don't read after the story!

Brawlhalla, paladins.

Here's a list on reddit with some free games with their ratings.

Void Pyramid is pretty cool also, 2d post apocalyptic rpg.

Warframe
Not so "hidden", but still hidden because I don't see it here often when people start a discussion about nice coop games.
Almost everything is reachable just playing...
I have almost everything I want/need, leveled up almost everything in the game and I still keep playing like the first days.
Also, the content is always fresh... We often get a update that brings a whole new world/mission/quest/items.

Totemori

Local-Multiplayer game. Great for 20-30 min during a couch-gaming evening. Simple and fun game.

Sang froid tales of warewolf

And Co-op games? Any recommendation?

I'd say Endless Sky deserves to be on this list. It's basically an open world, trading and exploration space sim with 2D top down graphics. You can become a pirate and capture enemy (read: other traders) ships :D. It's the spiritual successor to the old Escape Velocity series and a great game in it's own right. Unfortunately, I don't believe it's being updated anymore.

Here's the game series it's based on for the curious.

I'd also second Emily is Away and recommend PRICE :)
Edit: Ooh, if we can suggest non-Steam games, that I'd also go with Digital: A Love Story, though tbh it's more like a short book than a game.

Khimera: Destroy All Monster Girls is a really well made mega man like game. Feels like a full game despite being free. It gave me the same sort of enjoyment Shovel Knight gave me.

Saira was once a paid game, but the developer at some point decided to simply give it for free. It's a platformer set across multiple planets. I like it's simplicity yet it feels varied through the different planets. Each planet is like a mini metroidvania level. I like its sense of adventure and it generally has a chill vibe to it

Everlasting Summer is a visual novel set in a summer camp. I really enjoyed the fact that the protagonist is thrown into this summer camp and has no idea what's going on which is relatable for the player. I've never gone to a summer camp myself so it was fun to kind of retroactively be a teen in a summer camp meeting people. It also has lot's of endings.This game doesn't really seem hidden though as it's near the very top in ratings among the free games category.

Also happy to see some other free games I've enjoyed recommend by others. I'm going to check out some of the ones I'm not aware of.

Can we add non-steam games to this list? I see that many of you added what I had in mind, but many, many, many more suggestions!

Many thanks, OP!

I really liked https://store.steampowered.com/app/286100/You_Have_to_Win_the_Game/

I absolutely adored MANDAGON. https://store.steampowered.com/app/461560/MANDAGON/

https://store.steampowered.com/app/382490