Scott McCloud is author of Understanding Comics, a comic book about comics. He's an evangelist for comics as a valid literary form (as more than pulp and kids' stuff) and his admiring fans include a laundry list of superstar cartoonists.
If not for Scott McCloud, graphic novels and webcomics might be enjoying a more modest Renaissance. The flourishing of cartooning in the '90s and '00s, particularly comic-smithing on the web, can be traced back to his major writings on the comics form. The first, Understanding Comics, is translated into 13 languages, and along with Reinventing Comics and Making Comics, its playful and profound investigations are justly revered as something like the Poetics of sequential art.
McCloud coined the term "infinite canvas" -- for the new comics medium made possible by web browsers. He's an avid user of the medium: My Obsession With Chess was widely popular online, as was The Right Number. Back on the printed page, he wrote and illustrated Zot!, a colorful response to then-trendy grimness and gore in comic books. (He describes the book as "a cross between Peter Pan, Buck Rogers and Marshall McLuhan.")
He's the inventor of the game 5-Card Nancy.