Slender (known as Slender: The Eight Pages as of v0.9.7) is an independently-developed concept game based on the Slender Man mythos. The game centers around an unknown character being stalked and chased by the Slender Man within the woods while seeking eight pages scattered about various landmarks.
The game was created by Mark J. Hadley as an experiment. The experiment proved to be a success, and Slender gained mass amounts of popularity after going viral. It can be downloaded for free from Hadley's website.