Inside a Star-filled Sky is a procedurally-generated, recursive, nearly-infinite (it would take 2043 years of continuous play to reach the ultimate kill screen, but only 68 years to reach the first) tactical shooter by Jason Rohrer.
Gameplay is simple: move with WASD (or IJKL, or arrow keys) and use the mouse to aim and shoot. Move onto the up arrow pad to move up a level. But there's a twist. Every time you move up a level, you become the level you were just in. Collecting powerups on one level affects your abilities on the next (but not the current). You can even go down a level and enter one of your enemies, collecting powerups to change their capabilities once you emerge.