Slither.io

Slither.io is an free online video game the DanTDM made four videos on. It is somewhat difficult, as you can get killed in it very easily, something Dan soon realized while playing it.

Game Description
You are a snake/worm in a seeminly endless void with only one goal. Eat as many dots as possible and become the biggest snake/worm on the server. Take down other enemy snakes/worms, and they will drop all the dots they had eaten, allowing you to eat them and grow bigger.