In 1999 Fortune magazine identified the 25 most powerful interest groups in Washington, D.C. Here is the list along with links to their websites:

AFL-CIO (American Federation of Labor-Congress of Industrial Organization) [ranked #5]
American Association of Retired Persons (AARP) [ranked #1]
American Bankers Association [ranked #11]
American Farm Bureau Federation [ranked #21]
American Federation of State, County, and Municipal Employees (AFSCME) [ranked #22]
American Israel Public Affairs Committee [ranked #4]
American Medical Association [ranked #13]
Association of Trial Lawyers of America [ranked #6]
Chamber of Commerce [ranked #7]
Credit Union National Association [ranked #18]
Health Insurance Association of America [ranked #25]
International Brotherhood of Teamsters [ranked #23]
Motion Picture Association of America [ranked #17]
National Association of Broadcasters [ranked #20]
National Association of Home Builders [ranked #16]
National Association of Manufacturers [ranked #14]
National Association of Realtors [ranked #15]
National Beer Wholesalers Association [ranked #19]
National Education Association (NEA) [ranked #9]
National Governor's Association [ranked #12]
National Federation of Independent Business [ranked #3]
National Restaurant Association [ranked #10]
National Right to Life Committee [ranked #9]
National Rifle Association (NRA) [ranked #2]
United Auto Workers (UAW) [ranked #24]

Here are some other important interest groups:

American Bar Association

American Civil Liberties Union (ACLU)

American Conservative Union

Americans for Democratic Action

American Legion

The Business Roundtable

Christian Coalition

Drug Policy Alliance

Eagle Forum

National Abortion and Reproductive Rights Action League (NARAL)

National Association for the Advancement of Colored People (NAACP)

National Organization for Women (NOW)

National Resources Defense Council

National Right to Life

People for the Ethical Treatment of Animals (PETA)

Planned Parenthood

Sierra Club

 

And here are some rather "interesting" interests:

American Egg Board

Canine Companions for Independence

National Association to Advance Fat Acceptance

Snack Food Association

TV Turnoff Network

World Leisure

 

And if you are still feeling like you want to explore more interest groups, check out the Internet Public Library's "Political Advocacy Groups". If you want to get absolutely lost for days, then actually go to the Internet Public Library main page, it is like the world's biggest library.