Stuff to Look For

Keep an eye out for some of these sightings found in the show...-Gaz

Thanks to this excellent Jhonen Vasquez site.

Episode
Reference
The Nightmare Begins
The guy that does the "Old Kid's" voice is actually Jhonen Vasquez.
The Nightmare Begins
During the very beginning of the show where the alien announcer is shown on a screen, another screen floats behind him that shows the weird monkey-like thing on Zim's living room wall.
The Nightmare Begins
When Gaz opens the fridge a milk carton reads "Rob Hummel is Meat."
Walk of Doom
"Z?" shows up on a building.
Walk of Doom
Two gay men are standing and holding hands watching the Scary Monkey Show through a window in a store.
Walk of Doom
Filler bunny shows up near the steering wheel on the bus that ZIM rides on.
Walk of Doom
Two signs are on a building when ZIM and GIR are riding the bus. One reads: "Stop making babies" and the other one says "The goat whats you" I don't know if it was really supposed to say "The goat wants you"?
Dark Harvest
Filler Bunny is in a test tube, then broken in half by a ball that comes in through the classroom window.
Dark Harvest
When Dib is running through the hallways, there is a little sign on the wall that says "JV" which are Jhonen's initials.
Attack of the Saucer Morons
The guy that prophesized the coming of the "aliens" was Frank Conniff from Mystery Science Theater 3000, and also helped with writing ZIM.
The Wettening
Jhonen appears, sitting on his couch reading a comic, and his shirt reads: "Z?" He then chokes on a goldfish.
The Wettening
In the part where Jhonen chokes on a fish, the scary monkey thing that is on ZIM's wall is backwards behind Jhonen's aquarium.
Almost all of the episodes
Roman Dirge is seen walking down streets and sitting in restaurants occasionally.
Almost all of the episodes
Jhonen and Steve appear many times throughout the show.