Alter

This is one of the simplest games I've ever played. You control a red ship, the object is to move around and shoot all of the green ships. Once you do that, more green ships appear, you have to shoot them all as well. This repeats itself over and over again until you lose all your ships when they shoot you. Fun for the first few minutes, may get boring after a while due to its repetitiveness.

Download this for a quick time-killer, don't expect anything spectacular.