1 Popeye. Legendary inker Terry Austin is a huge Popeye fan, so much so that he sneaks Popeye into the background as often as he can.
2 Felix the Cat. At a local comic book store that Todd McFarlane frequented, he knew a fellow who carried a Felix the Cat doll with him at all times. The man was friendly enough, but would often tell McFarlane how he did not pick up any of his books because he was not a fan of superhero comics. Finally one day McFarlane asked him, “If I put Felix the Cat in my comics, would you buy them then?” He said yes, so McFarlane put Felix the Cat into his comics from that point on.
3 Jon Day. Jon Day was a childhood friend of Erik Larsen. When Larsen was just starting out, he drew Day into a small independent comic book that he was drawing. Jon apparently got the impression that Larsen was going to put him into every comic Larsen drew. Larsen figured, what the heck, and Day has appeared in pretty much every comic Larsen has drawn since then. He is a slight, balding fellow with glasses and a mustache. Keep an eye out for him in any given issue of Larsen’s Savage Dragon comic book!
4 Mystery Science Theater 3000. Artist Todd Nauck is a big fan of the television series Mystery Science Theater 3000, and he tries to work in a reference to the show whenever he gets a chance, even if it is as simple as writing “MST3” on a sign in the background of a panel. When he can, he’ll sneak the robots from the show into the background, too.
5 Barney Rubble. John Byrne is probably the king of sneaking little hidden jokes into his comic books. Perhaps his most famous Easter egg is the tiny drawing of Barney Rubble he would make whenever he had to draw actual rubble. Eventually, when the gag became too well-known (and therefore no longer a “hidden joke”), Byrne stopped drawing Barney in rubble. I would not be surprised if there are new jokes that Byrne is hiding in his current work that no one has discovered yet!