Dots
This game is called “Dots“. It’s very basic. No complicated how-to’s, tutorials, levels, or scores. I need to connect two or more dots (in same color). I can do it horizontally  or vertically. But not diagonally. And, that’s it. It’s about connecting.
This simple game is developed by Betaworks, available for free.
Anyway, according to the statistics, this simple game has been downloaded more than 1 million times in one week, and 25 million games has been played.