I've found quite a few funny or awesome mix up or parody videos which combine the musical themes or art style of 2 animes and mix them together, for example this one which i just found and i think is awesome, which is the Nichijou opening with characters from the Madoka universe
(I love how it even has the characters from the manga :3)
or the Durarara! opening with characters from Steins;Gate:
There's even ones that mixes anime with things that aren't, such as the Durarara! ending theme with Harry Potter characters, or the the Higurashi Kai opening theme with Final Fantasy 7 characters.
Anyone else got any to share? i love these things :D