Purloined from BUSLIB:
Daryl Cagle's Professional Cartoonists Index. Also here.
Editorial Cartoons (from Yahoo). Also has links to over 60 comic strips.
AAEC - Today's Political Cartoonists, National and International, from the Association of American Editorial Cartoonists.
USA Today Editorial Cartoons.
Also there are sites for individual cartoonists, for individual publications and for subject/topic collections of editorial cartoons.