While there are many IO multiplayer games based on worms, Slither.io was one of the first. While other multiplayer games have tried to reinvent the formula, Slither.io, also known as Slitherio, remains one of the best in the genre due to its tight controls and great graphics.
Dublonsio, the newest in a trend of addicting .io games, features a fun pirate-themed slant to the work-your-way-to-the-top multiplayer format.