Media: mp3
Who: Danger Mouse, The Beatles (The White Album), Jay-Z (The Black Album)
When: 2004
What: An album created by mixing together The Beatles and Jay-Z. Perhaps one of the most famous mash-ups to date due to the legal wrangling that ensued its release. The online delivery of The Grey Album and its popularity raised copyright issues regarding music downloads, sampling and intellectual property in a digitized age.
Links: site covering history and media coverage of The Grey Album, Danger Mouse's official site, wiki mashup article

Coincidently, I've made this post on the 3 year anniversary of "Grey Tuesday"!