Jigoku Shōjo

A thriller Anime, also called Hell Girl.

The Anime is about Enma Ai, a mysterious girl who has the duty to collect human souls and send them to hell.
In a page online at midnight, you can write the name of the person who you want to revenge, you send the name, and Enma AI appear and give you a doll with a red ribbon, and when you untie the ribbon, the person will be murdered by Enma.
But when your revenge is finished you go to the hell too.