Organizations:

Private Citizen – http://privatecitizen.com
Privacy Rights Clearninghouse – http://www.privacyrights.org/marketing.htm
Junk Mail Stopper – http://www.junkmailstopper.com/
StopJunk.com – http://www.stopjunk.com/
StopJunkMailNow.com – http://www.stopjunkmailnow.com/
JunkBusters – http://www.junkbusters.com/

Information:

Computer Professionals for Social Responsibility – How to get less junk mail
The war on junk mail
Electronic Privacy Information Center page on privacy and consumer profiling
Form 1500 – Stop Junk Mail Cold
Kuro5hin – How to stop receiving credit card offers
Michael Worsham – Junk Mail : Unsolicited Advertising that Pollutes Our Planet and Invades Our Privacy
MSN Money – How to fight spam, junk mail, and sales calls
Native Forest Network – resources for junk mail reduction
A write up by the City of Los Angeles on junk mail
Privacy Rights Clearinghouse – Privacy Basics and Opt-Out Strategies
Reduce.org – Reduce the hail of unwanted mail
RecycleStuff.org’s page on junk mail
EcoFuture.org’s page on junk mail
Ocean State Free Net – page on junk mail
Counterexploitation’s page on junk mail