Dungeons & Dragons provides a subtle one in the 3.5 Dungeonscape supplement, where you can buy a bouncy ball... just like the one Xykon inscribed a symbol of insanity on. Rich Burlew was one of the authors.
BURLEW's RESTAURANT. Try an Order of the Stick with our famous pie!
Goblins: One of the filler pages consisted of a very crudely drawn stick-figure comic that was drawn in-universe by Fumbles. One of the other goblins reading the strip felt like he'd seen a stick-figure comic set in a universe which adhered to Dungeons & Dragons rules somewhere before.