Toonpedia. An exhaustive catalogue of American cartoons, both animated and print, ranging from the ubiquitous, to the classic, to the largely forgotten. It also includes a cool "Today in Toons" feature.

(A MeFi search shows that it's been linked in comments several times, but it's never been an FPP; I had never encountered it before tonight, when I had a pressing need to know the name of the show that Gleep and Glop appeared on.)