Slither.io is a game that also involves controlling a circular entity (a snake) and trying to eat other entities (glowing orbs) while avoiding collisions with other snakes. It has a similar minimalist and competitive gameplay style to Agar.io.
Diep.io is a game that also involves controlling a circular entity (a tank) and trying to shoot other entities (tanks, polygons, drones) while upgrading the tank's attributes. It has a similar genre of multiplayer online action and a similar gameplay mechanic of splitting and merging entities to Agar.io.
Osmos is a game that also involves controlling a circular entity (a mote) and trying to absorb other entities (smaller motes) while avoiding being absorbed by larger ones. It has a similar genre of action and a similar gameplay concept of mass and size to Agar.io, but it has a more relaxing and atmospheric aesthetic and sound design.
Spore is a game that also involves controlling a single-celled organism and evolving it through different stages of life by eating, mating, and mutating. It has a similar genre of action and a similar gameplay element of cellular biology to Agar.io, but it has a more complex and diverse gameplay and graphics and a larger scope of simulation.
Hole.io is a game that also involves controlling a circular entity (a hole) and trying to swallow other entities (objects, buildings, people, and other holes) while growing in size and competing with other players. It has a similar genre of multiplayer online action and a similar gameplay theme of eating and growing to Agar.io, but it has a more colorful and cartoonish design and a different setting of a city.