:Links:

:Animals & The Environment:

• American Humane Association
• American Society for the Prevention of Cruelty to Animals (ASPCA)
• Best Colleges' Biology Degree Programs Guide 
• Friends of the Earth
• Greenpeace
• Humane Society of the United States
• The League of Conservation Voters (LCV)
• National Audubon Society
• National Parks Conservation Association
• National Wildlife Federation
• The Nature Conservancy
• Sierra Club
• World Wildlife Fund (WWF)

:The Arts:

• The Academy of American Poets 
• Americans for the Arts
• Association of Writers & Writing Programs 
• August Wilson Center for African American Culture
• Cave Canem
• National Endowment for the Arts
• National Endowment for the Humanities
• The Poetry Foundation 
• The Poetry Society of America  
• Voices of Our Nations Arts (VONA) Foundation
• Writer's Relief 

:Civil & Human Rights:

• American Civil Liberties Union (ACLU)
• Amnesty International
• Catholic Charities
• Council on American Islamic Relations
• Human Rights Watch
• Southern Poverty Law Center
• NAACP
• National Center for Law and Economic Justice
• National Coalition for the Homeless
• National Congress of American Indians (NCAI)
• National Housing Institute
• National Urban League
• Start Sleeping Homeless Resources

:Health Care & Disability:

• National Alliance on Mental Illness (NAMI)
• American Cancer Society
• American Public Health Association
• Center for Science in the Public Interest (CSPI)
• Families USA Foundation
• WGU: Healthcare and Medical Resources for the Homeless

:Politics:

• Congress.gov
• Congress.org
• The Congressional Record (GPO)
• Countable
• C-SPAN (TV/Radio)
• Politifact
• Talking Points Memo
• U.S. Senate
• U.S. House of Representatives
• The White House

Did I miss anything? If so, feel free to share your own
favorite helpful links in the comments!