Many teachers use Calvin and Hobbes comic strips in lessons. There is now a Calvin and Hobbes search engine where you can type in your query — homework, reading, etc — and then get the text and, in many cases, a link to the strip itself.

You might also be interested in The Best Comic Strips For Students & Teachers — 2010 (And Earlier) and Part Two Of The Best Comic Strips For Students & Teachers — 2010.