Agar.io is a really original online game where you control a tiny sphere with which you have to try to absorb other smaller spheres. The problem is that there are many other players doing exactly the same thing at the same time, and if their spheres are bigger than yours, you'll be absorbed too.
Depending on the username that you use you can play with one ball or the other. If your name is Mexico, for instance, your ball can have the flag of Mexico; if you use the name Doge, you can play with the famous meme dog.
The game system is really simple: you can move freely by pressing your finger on the screen, and dodge bigger players while you try to absorb the smaller ones. If you have bad luck and you are absorbed, you can always start over.
Agar.io is an original and fun game whose gameplay is very similar to that of the excellent Osmos. The main difference is that this game can only be played online, and thus it's much more frenetic.