Eat and grow merry (and quite big) in Agario 3D (AKA Biome3D), a 3D variant of the popular Agario IO game.
Use WASD to move around (but do take note that for some reason D and A are inverted), or split with spacebar.
For anyone with performance issues, you can alter the game resolution with +/- or the FPS cap with < and >.
Agario has become quite competitive. There's a lot of strategy in both eating to grow bigger, and the art of splitting to catch smaller players and eat them, too. Just be careful not to split too much too quickly, or you could be swooped up by a larger player.