Well not sure that they're riffs exactly, but:
Moonlight Sonata-Beethoven
Capricho Arabe-Francisco Tarrega
Revolution Is My Name-Pantera
Blackened-Metallica
La Villa Strangiato-Rush (especially the solo)
One Nation-Deadhorse
mmmk...there's lots more, but that's plenty for here.
Nice choice on that Machine Gun track by Jimi btw.