Agar.io

Agar.io, also known as Agario, is a classic .io game that kickstarted the .io games industry. It involves growing a cell to become the largest on the map by splitting on enemies and eating them. Created by Matheus Valadares, known as Zeach on Reddit, and released in April 2015, this game was the first major game to use the .io extension, and the game that seeded the rest of the .io games.

Gameplay
In Agario, 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. Viruses will pop a player into many smaller pieces, and can be dangerous if there are hungry players nearby, an experimental virus, the mothercell, emits pieces of food occasionally, which can be increased upon feeding it. There is an option to cerate an account, which saves all experience and can eventually unlock skins through potions, coins, and more. Sharing a high score on social media can unlock exclusive skins.