There are a number of web comics that promote skeptical thinking and science; prominent examples are xkcd and Saturday Morning Breakfast Cereal. Recently, I discovered Zen Pencils which started in February 2012. The general theme of the Zen Pencils is to illustrate inspirational quotes from famous people. The most recent installment is Welcome to Science which quotes Phil Plait, The Bad Astronomer. The full quote comes from a speech Phil gave at a science fair in 2005. It captures well what the skeptic movement and science advocacy are about.
Skeptical comics are not a recent phenomenon. North Texas Skeptics has been publishing comics as part of their publications dating all the way back in the 1980′s. You can find the comics in the newsletters archived on this web site, and there is a collection of some comics on the Cartoons page.