I was thinking recently about some of the best examples of online comics (I'm thinking about single-strip comics rather than story-arc comics like Girl Genius).
One example would be xkcd's "Duty Calls," which perfectly summed up internet culture in a single panel. And obviously xkcd has a lot of potential examples.
Another would be Saint Gasoline's "The Allegory of the Trolley Problem Paradox," which is a nearly perfect philosophy comic.
What are some others?