I read about it on the Paper Pastries blog that I pretty much read daily.
You fill out a form with your name and address, tell them a topic you want a letter on and they send you a fun letter in the mail for free! If you can't think of a topic, they will pick one for you. I requested a letter last week on the topic "Mail Art"!
p.s. I'll let you know when I get my letter in the mail!