Gun Owners of America, 8001 Forbes Place #102, Springfield, VA 22151, 703/321-8585 — The no-compromise poltical action group on firearm and militia rights.
National Rifle Association — Defender of the Second Amendment, but a little weak on the rest of the Constitution.
Citizens Committee for the Right to Keep and Bear Arms (CCRKBA) — Another strong force for the RKBA.
The Lawyer's Second Amendment Society — Unites constitutionalist lawyers.
Second Amendment Foundation (SAF) — Tax-deductible companion to the CCRKBA.
Jews for the Preservation of Firearms Ownership — Defend Second Amendment from a Jewish perspective. But non-Jews are welcome to join, too.
Opencarry.org — Promotes open carry legislation.
Concerned Citizens Opposed to Police States (CCOPS) — Oppose abuses of power.
The Law Enforcement Alliance of America — Unites law enforcement professionals, opposes gun control.
Law Enforcement for the Preservation of the Second Amendment — Unites law enforcement professionals against gun control.
Citizens Of America — Oppose compromise on firearm rights.
Women Against Gun Control — Unites women who refuse to be helpless victims.
Madison Society — Focus on Second Amendment compliance through litigation.
Task Force Two — Firearms and militia rights political action group.
Law Enforcement for the Preservation of the Second Amendment — Unites law enforcement professionals.
Knife Rights — Defend right to keep and bear knives and other non-firearm weapons.
National Firearms Association — Canadian RKBA organization. 

Keep and Bear Arms -- For the Liberty of all people on this planet