Voar.io

Voar.io was the first .io game, and was created by Matheus Valadares, the creator of Agar.io. Voar was first released in 2014. The website has been down for years, however, and it is unlikely to ever go back up. There is no app associated with this game for mobile users, either.

Gameplay
The game involved creating spaceships that orbited a planet, and the ships would sometimes crash into each-other.