If you like horror games, you need to play SCP-087 immediately, a very small and free experimental indie game which takes the concept of delving into the unknown to unbelievably terrifying heights.
You can download it here:
http://www.mediafire.com/?5cadimaeo004aee
It's best not to know anything about it but if you're a little wary, you can see a video the developer made which goes through it here
www.youtube.com/watch?v=uiHQN1q8ln4 and you can read the story it's based off from the unique SCP Foundation horror site here
http://www.scp-wiki.net/scp-087.
But I'd really recommend not reading anything about it until you've played it. See if you can make it to the end without quitting out of fear. I couldn't do it.