[Senkusha]

I'd like to start a conversation that lists various crossovers that occur within anime.  For example:

In Negima! Magister Negi Magi, there's a scene where Nodoka gets a special book that can give her anything she wants.  There is an Ah! My Goddess reference in this episode "I wish for boy just like you..."  At least, I think that's the quote.  It's been a few years since I've watched this particular anime.  Maybe it's time I blow the dust off of it and sit down to spend some time with Negi and his class.

What other official anime crossovers do you know about?