Agario abs0rb.me is an extremely simple 2D action game in which you have complete control over a ball.
agar.tv is a 2D multiplayer tower defense game in which you are given a pickax to defend your gold mine against a whole lot of agario that also have pickaxes
Karnage.io is a 2D multiplayer cannon battling wargame.
Deeeep.io is an evolutionary game in every sense. The objective of the game is to eat bubbles and go up the food chain. As expected, starting out, the player is a small animal.
Superhex is a game in which you have to protect your territory from intruders. This is also a game that is played fast and multiplayer.