MMORPG

Adventure Quest Worlds: https://www.aq.com/

Battle Royale : 

• https://surviv.io/
• http://tanksmith.io

Club Penguin: 

• https://play.cprewritten.net/
• https://cponline.pw/

Dragon Fable: https://www.dragonfable.com/

Dungeons and Dragons: https://roll20.net/

Pokemon Showdown: https://play.pokemonshowdown.com/ 

RuneScape: https://www.runescape.com/splash

TL; DR eat people, get big: 

• https://agar.io/
• https://slither.io/

ToonTown: https://www.toontownrewritten.com/

Warframe: https://www.warframe.com/