I was reading over New Avengers 28 again and low and behold I just saw Morpheus and Death from DC/Vertigo commenting about how nice Doctor Strange's house was and its sad too see it demolished. And i seem to remember Azrael mentioning Jigsaw in Knightfall.
So tell me more cross company reference or appearances.
because Wong is, hands down, the single greatest character in comic books. Ever.
Death of the Endless showed up in the Hulk, handing Rick Jones a brush.
(haha, get it? brush with death)
a female John Constantine showed up in "Pryde and Wisdom."
Jim Shooter (not a character, but cool nonetheless) got the sh-- kicked out of him by Guy Gardner while he ranted and raved about how he was going to use the Sunspot (Starbrand) to create a new universe (the New Universe).
I know there are others, just can't think of them right now.