Mashups are, according to a Wikipedia entry which needs some editing and discussion around its content, almost a decade old. (Music sampling has existed for decades.) The genre continues to evolve, with stand-outs going viral and their creators/DJs gaining worldwide recognition.
"Carpenter's Wonderwall" is one such example: a song/video blend created by DJ Mark Vidler of Go Home Productions. It starts with Oasis's "Wonderwall" and is joined by The Carpenters' "We've Only Just Begun," bridging hits from different musical eras. It's brilliant on so many levels — enjoy.