These were brought to my attention through the Bad Astronomer himself, and are more than worth a read. I've been known to write my fair share of 'Open letters to...' various organizations, but these are far better done than anything I could come up with.
Dear Media, from Science
Dear Homeopathy, from Science
Dear Astrology, from Science
Edited on 4Feb to add: Ok, so I only followed the links provided by Dr. Plait, but there are several more of the Science letters, find them here... I highly recommend Dear Advertising, from Science