Which free online co-op games do you recommend for 2-3 players?

Not these standards like Rocket League, fortnite, CSGO, valorant, lol, Osu, …

Stop playing games that you don't have on your PC yet

Here is some list of games that are not so mainstream:

War Thunder (World War 2 - Present time: tank, aircraft and ship battles)
Crossout (Futuristic Mad Max DIY vehicle battles)
Dota 2 (MOBA, can be played like lol)
Heroers of the Storm (MOBA in the Blizzard Universe)
Heroes and Generals WWII (good Battlefield 5 alternative)
Roblox (cringe, but you can do a lot)
Totally Accurate Battlegrounds (Battle Royale, but with insane weapons)
Team Fortress 2

Brawlhalla