mattyfez, Human Fall Flat is cross platform albeit not Rocket League levels of excitement. Fortnite is cross platform in certain directions if you don’t mind getting pwned by pre-teens.
Fortnite: PS4, Xbox One, Switch, PC, mobile
Dauntless: PS4, Xbox One, PC, Switch
Rocket League: Xbox One, Switch, PC, PS4
SMITE: Xbox One, Switch, PC, PS4
Paladins: Champions of the Realm: Xbox One, Switch, PC, PS4
Realm Royale: Xbox One, Switch, PC, PS4
Call of Duty: Modern Warfare: Xbox One , PS4 , PC
Call of Duty: Warzone: PS4, Xbox One, PC
Brawlhalla: Switch, Xbox One, PS4, PC
Super Mega Baseball 2: PS4 , Xbox One, PC, Switch
Fantasy Strike: PS4, Switch, Mac, PC (including Linux)
World of Warships: Legends: PS4, Xbox One, PC
Minecraft: Xbox One , PS4, Switch , PC , mobile
Power Rangers: Battle for the Grid: Switch, Xbox One, PS4, PC
Partial Support
Note: A slash indicates separate cross-play support for different platforms
DC Universe Online: PS4, PC
Aragami: Xbox One, PC, Switch / PS4 , PC
Ark: Survival Evolved: Xbox One , PS4 , PC, Switch
Astroneer: Xbox One , PS4 , PC
Black Desert Online: PS4, Xbox One
Blobcat: Switch, PC
Chess Ultra: Xbox One, Switch, PC/PC, PS4
Deep Rock Galactic: Xbox One, PC
Dick Wilde 2: PS4, PC
Disc Jam: Switch, PC/PC, PS4
Eve: Valkyrie: PS4 , PC
Final Fantasy XIV: A Realm Reborn: PS4 , PC , Mac
Full Metal Furies: Xbox One, PC
Games of Glory: PS4, PC
Guns of Icarus Online: PS4, PC, Mac
Happy Wars: Xbox One, PC
Hearthstone: PC, Mac, iOS, Android
Hero Siege: PC, Mac, mobile, Switch/PC, Mac, mobile, PS4
Hex: PS4, PC
Hover: Xbox One, Switch, PC/PC, PS4
Kabounce: PS4, PC
Mantis Burn Racing: PS4, PC/Switch, Xbox One, PC
Mortal Kombat 11: PS4 , Xbox One , Switch
Mushroom Wars 2: Switch, PC, Mac, mobile
NBA 2K Playgrounds 2: Xbox One , PS4 , Switch , PC
Next Up Hero: Xbox One, Switch , Mac, PC
Overload: Xbox One, PC/PS4, PC
Phantasy Star Online 2: PS4 , PC
Pinball FX 3: PS4, PC/Switch, Xbox One, PC
PlayerUnknown’s Battlegrounds: PS4 , Xbox One
Pox Nora: PS4, PC, Mac
Pure Chess: PS4 , PC, 3DS
Rec Room: PS4, PC
Riptide GP Renegade: Switch, PC/Xbox One, PC
Roblox: Xbox One, PC, Mac, mobile
Siegecraft Commander: PS4, PC/Switch, Xbox One, PC
Sniper Elite V2 Remastered: Xbox One , PS4 , Switch , PC
Spacelords: PS4, PC /Xbox One, PC
Sports Bar VR: PS4 , PC
Star Trek: Bridge Crew: PS4 , PC
Street Fighter V: PS4 , PC
Square Heroes: PS4, PC
Super Dungeon Bros: PS4 , PC, Mac/ Xbox One , PC
Trailblazers: PS4, PC, Mac/Xbox One, Switch, PC
Treasure Stack: Xbox One, Switch, PC
Tooth and Tail: PS4, PC, Mac
Ultimate Chicken Horse: PS4, PC, Mac/ Switch , PC, Mac
War Thunder: PS4 , PC, Mac/Xbox One
Wargroove: Switch , PS4 , Xbox One, PC
Weapons of Mythology: New Age: PS4, PC
Werewolves Within: PS4 , PC