I thought it'd be fun to make a thread cataloguing all of the non-Marvel properties that're a part of the Marvel canon.. So here we go!
I'll start y'all off with an example:
Elric of Melinboné (and the rest of the Moorcock Multiverse) had a canonical crossover with Conan during the Roy Thomas series (a series that takes place on Earth-616)... Which means the Moorcock Multiverse is technically a part of Marvel's canon.