Geometry Dash

You know, some free online games seem very easy. At least in the graphics. But that is misleading. At least, in this game, Geometry Dash. It is as simple as can be in design and appearance. But it's fiendishly difficult to play because microseconds sometimes define win or loss. The essence of the game is that you play with a cube, which slides through the levels. We don't know exactly how it propels itself forward because there's no obvious power source or moving parts to be seen. During sliding, he has to jump past various obstacles (such as spikes, which can kill) and ascend/descend through terrain, which can cause a cube to stay too long, which might be enough to kill him.