Nytimesimages.com
http://www.nytimes.com/pages/cartoons/
Political cartoons from the New York Times.

Daryl Cagle's Professional Cartoon's Index: PoliticalCartoons.com
http://cagle.msnbc.com/
Editorial cartoons updated daily from cartoonists around the world.

Time.com: Cartoons of the Week
http://www.time.com/time/cartoons/

USA Today: Editorial Cartoons
http://www.usatoday.com/news/opinion/cartoons.htm

Yahoo! News: Editorial Cartoons
http://news.yahoo.com/news?tmpl=index2&cid=1078
Click on the first cartoon for more.