Not counting ElseWorlds and the like. Examples: Green lantern/Aliens. Batman/Spawn. Batman/Judge Dredd. Wolverine/Badrock. There are many more. So do any of these count? Does Spawn live in the DCU? Did Superman really fight Aliens?
not much of a spoiler , but the C.A.T.s respond to a emergency distress call from another hero team stormwatch. lets just say stormwatch had some team members who "retired early ", their demises stayed after the arc was done. if u count wildstorm different from DC , captain atom and mr majestic had crossovers that stuck.
outside of traditional superhero stories the hack / sash series had crossovers that are part of continuity as well , they had the heroes go against evil ernie, chucky, and bomb queen
Some are despite people's denials. batman\punisher Lake Of Fire was canon in the DCU.
Gladiator vs Supreme is cannon as it has been mentioned by both company's the fight happened .
Wow, I didn't realize either of those were canon. The Batman/Punisher crossovers were awesome.
@wildvine : Anything crossed over with Savage Dragon is considered canon by Erik Larsen, including the Superman/ Dragon crossovers. The most he was able to acknowledge them in the Savage Dragon series though was showing Angel running around the house in Superman's cape. Something he'd asked Supes for in the crossover. (As far as I know, it's never been mentioned in a DC book.)
Also, I think any company crossovers between creator owned projects are generally considered canon, even if it's unspoken. Otherwise, what's the point? Plus, they don't have to edit the trade paperbacks that way.
Also again, I think the Invincible appearance in Marvel Team-Up #14 is considered canon, and was reprinted in both the MTU and Invincible trades (I think).
I was under the impression that the creation of Access was meant to provide a canon explanation as to crossovers between Marvel and DC, with him being the original character shared between the two companies. Since the whole incident a while ago, there's no telling whether the recent DC reboot evicted Access from DC continuity or not, which kind of makes Access entirely Marvel's character, but also lacking any purpose for existing.
I'd like to think that the crossovers being canon was true. Since Continuity is kind of a big deal in comics, having people who only focus on such crossovers having a continuity to follow would be interesting, instead of just them getting one off stories that always start with "Dafuq are you?"