Bach's (?) "Jesu, Joy of Man's Desiring" was done up by Apollo 100 as "Joy". Not a guitar piece, though, it was definitely a keyboard-fest!

And "Tocatta And Fugue in Dm" has been subjected to some very diverse treatments, as has "In The Hall Of The Mountain King".

Richard Straus might have some interesting comments on the things people have done with the first movement of his "Also Spract Zarathustra" (sp?).