I could have sworn I read sometime that one of the crossovers actually had some connective tissue to DC Comics in general, like some plotline was followed up in a just-DC comic. So I was wondering if that was true.
I'm only really aware of 3 crossovers.
1976: Superman vs. The Amazing Spider-Man - Takes place in an Elseworld inhabited by both characters.
1996: DC vs. Marvel
2003: JLA/Avengers - This, I don't believe, referenced DC vs. Marvel, and had characters meeting for the first time again. Tell me if I'm wrong.