Watch some video examples of Music mashups.
Here is an excellent introduction to Mashups. It starts with a mix of classical and rap music, presents a timeline of mashup development, and concludes with a description of the ethical debate that surrounds mashups. Do artists have the right to use the original music and transform it into something else?
A Mashup of Green Day and Oasis Music
Shakira, Britney and The Bee Gees