Great musical reinventions
"Mambo No. 5"
"We Will Rock You"
"Oops, I Did It Again"
And, as a bonus, a mashup of "Tainted Love" featuring both Max Raabe and Marilyn Manson. Alas, not as good as the concept, or as good as the mashup of Manson's "1996" with the Powerpuff Girls Theme song from a few years back.
Max Raabe links courtesy of bruceb