I just went on to Amazon.com and was presented with PostSecret : Extraordinary Confessions from Ordinary Lives as a book selection (I'm not sure why, maybe it's just a popular book at the moment).
It sounded interesting for some reason, so I tracked it back to the website that is the source of the book's material and to a news article by The Guardian about the project.
The original instructions (+ a postcard) from the creator, Frank Warren, were as follows: "You are invited to anonymously contribute a secret to a group art project. Your secret can be a regret, fear, betrayal, desire, confession, or childhood humiliation. Reveal anything -- as long as it is true and you have never shared it with anyone before. Be brief. Be legible. Be creative."
The Guardian had this to say:
For the anonymous contributors to postsecret.com, for the millions visiting it, and for Frank himself, the most profound realisation that comes from peering so deeply into the troubled souls of the postcard-senders is simply this: that the things that make us feel so abnormal are actually the things that make us the same.
I found a bunch of other "confessional" style websites with associated books, but this one has a better feel than others, mainly because requirements include creativity and brevity (and the real-life time and effort to create the card).
I thought it was very interesting.