Potential Crossovers?

[Skuld-chan]

I love crossovers.  Mostly, I find them in fan-fictions that I read, and many of them are quite interesting.  I know that I've seen in Star Trek there are several "Star Trek (related) crossovers), where characters from other series are in a series they don't belong to, but are there any anime where crossovers occur?

One I can think of, and I guess it's not really a crossover is in Negima, Nodoka has her book that she starts asking for "I wish for a teacher like Negi...."  Obviously, a rip from Ah My Goddess, but technically a crossover.

Are there any actual crossovers though, in official anime?

[ZandraJoi]

While I haven't read anime with crossovers, I will go by how I view it in TV shows. I don't care for it. Unless one is already reading (or watching) both stories, it can get quite confusing.