It's the (apparent) end of the world. What are you gonna do? Run like hell, I suppose. This is the plot of Canabalt, a Flash game in which you have one goal, one purpose: run, as far as you can, before dying.
You will run - in fact, there's no way to stop running, though stumbling over random boxes and office chairs will temporarily slow you down. You will die - death is inevitable, either by falling off the roof of a building to the street below or being turned into a fine mist by some newly detonated alien explosive. The only thing you can control is how far you can get with your remaining time on what's left of this earth by jumping from roof top to roof top, across the arms of cranes and through plate glass windows.
Flocks of birds flee at your incessant approach. Menacing mecha-alien silhouettes loom on the horizon. These are the last moments of your life; it's up to you as to how long you can make that last.