Agar.io is one of those games that seems dull on the surface, but absorbs* people's time like crazy once they give it a try. In case you haven't played it, the premise is you are a small cell that absorbs other, smaller cells when you come into contact with them, growing and expanding with each cell eaten. Other players are also attempting this, and it's fairly common for players to grow large enough to each one-another.
Leave it to SethBling to come up with a vanilla version of this popular game: as the video shows, slimes take the place of cells, but the overall objective is about the same. You can try it right now, and it is 100% vanilla. Yes, like its web-based counterpart, you can play with (and absorb) friends too!