Agar.io

Before "io games" became a genre, there was Agar.io. Released in 2015 by Brazilian developer Matheus Valadares, this deceptively simple cell-eating game spawned an entire category of browser multiplayer experiences.

You're a cell. Eat smaller cells to grow. Avoid bigger cells that want to eat you. That's it—yet somehow it's impossibly compelling. The real depth emerges from splitting: launch half your mass forward to capture prey, but now you're vulnerable, smaller, a target yourself.

Servers host dozens of players in chaotic free-for-all battles. Team modes add coordination. Leaderboard chasers fight for server dominance while casual players enjoy quick snacking sessions. Cosmetic skins let you personalize your cell; clan tags identify organized groups hunting together.

The game that launched a thousand clones remains the definitive .io experience. No downloads, no accounts—just agar.io and instant action. Mobile versions on iOS and Android bring the same gameplay on the go.