Being a big fan of both DC and Marvel, I've read nearly all of the intercompany crossovers produced where characters from both fictional universes met up. Some stories have characters from one universe travelling into another, while others portray both realities as the same. The thing is though, there were two particularly big crossovers that impacted on both realities - they were the DC vs Marvel event, which introduced the character of Access, and then there was the much later JLA/Avengers crossover, some of the events of which were later referenced in the regular DC universe. But now I'm not so sure if the DC vs Marvel event is still canon anymore, as JLA/Avengers makes absolutely no reference to it and the characters from the DC and Marvel 'verses appear to become aware of each other for the first time. Access isn't mentioned either, and someone so pivotal to both 'verses should logically have put in some sort of appearance.
EDIT: Access briefly cameoed in an issue of DC's Green Lantern, sans any Marvel characters.
EDIT: Access briefly cameoed in an issue of DC's Green Lantern, sans any Marvel characters.