Action Network
Select from dozens of organizations and receive email notices of current action opportunities.  Provides an online method to send feedback to government officials.

American Civil Liberties Union
Subscribe to alerts and use their online form to contact your representatives on matters concerning our constitutional liberties.

Electronic Frontier Foundation
Subscribe to alerts and use their online form to contact your representatives regarding online communication and privacy issues.

Electronic Privacy Information Center
Subscribe to a newsletter on emerging civil rights and privacy issues.

Public Citizen
Founded by Ralph Nader 30 years ago, this site has a wealth of information on a wide variety of topics including health care, campaign finance reform, product safety, and more.  Some pages also include online forms you can use to contact your representatives.