I just opened
Letters to Breathe a site to submit unsent letters to anyone, about anything, mean letters, love letters, angry, hurt, etc.
Most letters are signed with a pen name to remain anonymous BUT you can submit a letter with a link back to your blog eg. From, Tony of (tonys blog)
All I ask is that the letter you submit not be just a letter thrown together to get a link back, it needs to feel real to the reader, I am sure you can come up with a letter to someone that has hurt you or pissed you of, I will not accept letters that appear to be written just for the link.
Anything that is emotional and written like a letter works- see some of the current letters for examples.
Even if you want to do it anonymously and not get a link back it works for me- I just want some submissions LOL