Mashups are fascinating.
One of my favorites is a mashup of Amy Winehouse's "Rehab" with the Four Tops doing their Sixties Motown hit "I Can't Help Myself (sugar pie, honey bun)".
But the ultimate mashup IMO so far is "Feed the Animals" by DJ Girl Talk which is an almost hour long mashup of what must be a couple hundred songs-ranging from pop to classic rock to the latest rappers of a couple years ago. Blackstreet's "No Diggety" overlaps and beat-matches the Beachboys- who blend into Twisted Sister, who merge with SaltNPepa, and so on.. Absolutely mind blowing.
One of my favorite moments is a sample of an instrumental passage from the song "New Soul" by that Israeli girl singer (whose name escapes me)- the song about how "I am a new soul fallen into this strange new world..." is superimposed upon a rapper (Lil Wayne?) raping about how "I got drunk, I got krunked, I got f****d up!" (the ultimate in innocence juxtaposed with the ultimate in debauchery, but both songs are in the same key and have the same beat so they fit together hand and glove).