Here are several:
An Open Letter to Package Manufacturers
An Open Letter to Mr. Epson
An Open Letter to the Bicycle Thief
An Open Letter to the Free Press
An Open Letter to Maureen Dowd
An Open Letter to Moviefone.com
An Open Letter to Cold Stone Creamery
An Open Letter to Sprint
Alas, some notable entries, like my complaints to Entenmann's bakeries I can't track down, nor my product suggestion letters to Tasty-kake bakeries. Nor my advice to the Philadelphia Zoo, RE: the zooballoon. Posterity can only suffer for their lack.