So you want to write a letter..... but for what purpose? To Complain? To give a Compliment? To express a Concern? Usually it's one of the "Three C's."
Think of all the things you come into contact with every day and you should have no trouble picking someone to write a business letter to. Was your burger undercooked (Complaint)? Did you get especially great service from a specific person at a business, that makes you want to return as a customer (Compliment)? Do cars speed in your neighborhood, and maybe there should be speed bumps (Concern)? Your only limit on topics to write about is your imagination.
But whatever your message, remember to slow down and think about your letter's desired outcome. Venting is nice, but wouldn't it be nice if the reader actually took your comments seriously and responded? Or better yet, changed the way they do things to your benefit? In life, HOW you say things is just as important if not more important than WHAT is being said.