The great-granddaddy of them all, Agar.io is an eat or be eaten game of cellular survival taking inspiration from the behavior of microbes on top of the agar medium of a petri dish, hence the name. 

Players start small, zipping around the map and eating food to gain size. Eventually, you grow large enough to engulf other player cells, resulting in a deadly cat and mouse game as you try to lure other cells into your grasp while evading larger swarms of players that can engulf you. Simple but tense gameplay made it a viral sensation, as it was even featured in an episode of House of Cards.