Here's an interesting game made by two friends of mine called East.
You go through it by going, ironically, in the opposite direction. You go to the left, platforming, collecting coins, and stomping on enemies. All of this increases your ever-decreasing score.
As the score decreases, the scenery and the enemies (as well as the music) change. They go from relatively normal to...well, you'll see.
Also, there is only one way to end the game.
There is a sequel in the works titled West.
A video of the game: