Agar.io was the first .io game that popularized the trend of these .io games. Created by Matheus Valadares, known as Zeach on Reddit, and released in April of 2015, this game was the first major game to use the .io extension, and the game that seeded the rest of the .io games.
In Agar.io, players play as cells in a petri dish. The object of a game is to become the largest cell by moving around eating agar pellets, as well as smaller players. Players can also divide into smaller cells and eject mass.