Depending on what games you like, I might be OK with sharing my ID.
- XCOM 2 and XCOM Enemy Unknown/Enemy Within
- Civilization 5, Beyond Earth and soon to be Civilization 6
- Endless Legend, Endless Space and Dungeon of the Endless
- Age of Empires II HD
- Borderlands 2 and Borderlands: The Pre-Sequel
- GTA V
Those are the only multiplayer games I own on Steam.
The rest like Dishonored, Fallout: New Vegas, Fallout 3 and Fallout 4, Deus Ex: Human Revolution, Bioshock and Bioshock Infinite, STALKER: Shadow of Chernobyl, Skyrim, Morrowind and so on are single player only.
...
I also have a few games on GOG, but I'm not sure if it supports multiplayer. The Witcher 3, KOTOR 1 and 2, Terraria, Satellite Reign, Amnesia: The Dark Decent, Metro Redux, Deus Ex GOTY, System Shock 2, Thief 2, Star Wars Battlefront 2 and a few others.
Oh, and I've a few non-official games, like this
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It supported in-game trading and online battling with other people, but no longer does as the game has been removed by the creators due to Nintendo's pressure. It was not an official release, but I was quick enough to download it and still have the files as a backup
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I love me some Pokémon.