Social Studies

One World Classroom
National Geographic  

art

The Getty
The Smithsonian
WebMuseum, Paris

music

The Kennedy Center
K-12 Resources for Music Educators
The National Association for Music Education
Musica International

business and economics

American Stock Exchange (AMEX)
Bank of America
BizWiz - The Internet's largest business network
BusinessWeek
Charles Schwab
Stat-USA - A trusted source for federal business, trade, and economic information
U.S. Patent and Trademark Office
The Wall Street Journal

civics, government, and politics

Central Intelligence Agency 
Colorado Bar Association
Colorado.gov - The official site of the state of Colorado
The Library of Congress
U.S. Department of State
United States House of Representatives
The White House

history

Colonial Williamsburg
Gettysburg
The History Channel
Living the Legacy:  The Women's Rights Movement of 1848-1998
Martin Luther King Jr.  - Paper projects from Stanford University
National Women's History Project (NWHP)

humanities

The humanities are those academic disciplines which study the human condition using methods that are largely analytic, critical, or speculative, as distinguished from the mainly empirical approaches of the natural and social sciences. Conventionally the humanities include the classics, languages, literature, music, philosophy, history, religion, and the visual and performing arts. Additional subjects sometimes included in the humanities are anthropology, area studies, communications and cultural studies, although these are often regarded as social sciences.