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.
American Political Prints 1766-1876 provides a browsable & searchable collection containing hundreds of political prints, originally published in Harper’s Weekly, presented by the Library of Congress in chronological order.
Oliphant's Anthem features numerous works of the 1966 Pulitzer Prize winning editorial cartoonist Pat Oliphant.
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.
The Political Cartoons and Cartoonists section of boondocksnet.com includes early political cartoons by Thomas Nast and Honoré Daumier and portrayals of Theodore Roosevelt, Uncle Sam and woman suffrage. Their Historical Graphics Gallery includes The Political Art of Dan Beard, Advertisements from the Spanish-American & Philippine-American Wars, and Stereoscopic Visions of War and Empire.