PostSecret is a project/blog created by Frank Warren in 2005. People mail anonymously a postcard which they artfully decorate, portraying a secret they have never revealed before. Franks posts these postcards on his blog, which is one of the most visited.
Frank has also published three books with PostSecrets. Lately, he made a mini-movie on the project, very worth watching. He also likely 
saved a few lives, and collaborates with the Hopeline. People’s secrets are moving, beautiful and sometimes scary. This is the Internet at its best.
A full story on PostSecret may be read in its Wikipedia’s entry. A lot of PostSecret postcards are stored as images in Flickr (with Kawaface’s photos).
