Agar.io is the original .io game that started the craze! You play as a cell, and absorb other smaller cells and players to grow! Super addicting game.
To play Agar.io, you simply use your mouse to move around, and you try to eat other players. You can hit spacebar to launch half of your mass forward, or you could use the "W" key to share mass with other players.
In agar.io, just try to not get cornered, and use the walls of the map to your advantage. Stay away from the big guys!