Library of Congress:
Oliphant's Anthem features numerous works of the 1966 Pulitzer Prize winning editorial cartoonist Pat Oliphant.
July 2009 update: Polisource was notified of the following regarding an outdated link (now removed) to American Political Prints and various political cartoons that the Library of Congress makes available online.
American Political Prints 1766-1876 is no longer separated out in the Prints and Photographs Online Catalog. Most of the cartoons published in the book are available here: http://lcweb2.loc.gov/pp/apphtml/appabt.html, but there are more cartoon categories in the Prints and Photographs Online Catalog: http://www.loc.gov/rr/print/catalog.html. The Library of Congress has done several exhibitions on political cartoons, not just Oliphants Anthem. A checklist of most of them is here: http://www.loc.gov/rr/print/swann/swann-exhibits.html, with the exception of these two: http://www.loc.gov/exhibits/cartoonamerica/ and http://www.loc.gov/exhibits/treasures/herblock-home.html.
PoliticalCartoons.com provides lists of editorial cartoonists in several catagories. A collection of cartoons by many of these artists is available and updated daily. You can even send someone an e-greeting card with the cartoon of your choice on it! You can link to the cartoonists' websites and get the email addresses of those without websites. Lesson plans are available in the Teacher's Guide, for using the cartoons in the classroom.
Puck's Homepage: Uniting Mugwumps and the Masses - This "analysis of cartooning as well as Gilded Age political culture" discusses how the cartoons in Joseph Keppler's satirical magazine Puck conveyed the liberal viewpoint during the 1880's. There are several images (a few with extensive explanatory notes) of the work of Keppler, Bernhard Gilliam, and James A. Wales. Includes A Brief History of Cartoons.
"Red Scare is an image database about the period in the history of the United States immediately following World War I...approximately from the Armistice in November of 1918 to the collapse of hyper-inflation in mid-1920." All images (mostly political cartoons) are public domain. The political cartoons are also available here, on Polisource.
Art to the People provides a collection of political prints and illustrations from the 1880s through the 1930s, featuring brief profiles and works of Walter Crane (Great Britain), Theophile-Alexandre Steinlen (France), Albert Hahn (Netherlands), Frans Masereel (Belgium), and Gerd Arntz (Germany). Browsable by artist and symbol (light, labor, capital, death, and woman). This is an online version of the 1997 National Trades Union Museum exhibition in Amsterdam, organized by the International Institute of Social History.