Slender is a free downloadable game that only takes a few minutes to play through. It takes place in a forest and essentially all you have to do is find 8 notes that are scattered around...and that's it. Find the 8 notes and you're done.
There's obviously more to it then that but saying any more would spoil it. It's best to play the game without knowing much about it. It's definitely an experience, one that's amazingly atmospheric and well made despite it's limitations.
Here's the download link to anyone that wants to give it a try.
http://www.mediafire.com/?vql3a6fbjq0qbbv
It'd probably be best to play it in the day if you're a little faint of heart.