Paper.io is an awesome io game which has been ported onto the web from it's original mobile version.
How to Play Paper.io
To play, use your arrow keys to steer your player/line, much like the way you would if you were playing splix.io or nafk.io.
The strategy is the same as in splix. Try to conquer as much territory on the map as you can, without letting anyone run into your line.