Open Letters are letters sent to  people, organizations, and groups that are also open to viewing by other people and groups.  I like to think they can be very effective from their power to put people on the spot and in the spotlight, especially as they know a wider audience is viewing.
Editorials can also be effective in stimulating action and change  in their viewers.  A good resource for both of these categories would be:
The Media Awareness Project
 http://www.highexistence.com/an-open-letter-to-president-obama-end-the-war-on-drugs/
http://www.drugwarrant.com/2013/05/an-open-letter-to-marijuana-prohibitionists-and-so-called-third-way-ers/
http://stopthedrugwar.org/files/coalition.pdf