It's every comic fan's dream to see our favourite heroes from Marvel and DC meet up. Message boards have been completely overrun with posts arguing over who would win in a fight between Batman and Captain America or how cool it would be to see Superman stop Galactus.
Unfortunately though, thanks to the rivalry between owners and the minefield of paper work needed to organise it, crossovers between the two companies are few and far between.
However, while the Marvel and DC bigwigs may be constantly at each others' throats (as is wont to happen when you're the kind of person for whom the term "market shares" isn't a natural alternative to sleeping pills), things have always been a lot friendlier between the actual people who write the comics.
More importantly, the people writing comics are probably even bigger nerds than the one's reading them so of course they want to see their favourite heroes cross over.
To that end, there have been multiple times when writers at both Marvel and DC have skirted the rules to have the two universes meet in a way that won't send lawyers running.