I've been really grooving on Fissunix lately. Especially his Marvin Gaye vs Led Zepplin mashup. One of the best i've heard in awhile. Here are two great one's from him.
Fissunix - "Ain't No Stairway High Enough To Hip Hop Heaven"
http://vimeo.com/19142214 Marvin Gaye "Ain't No Mountain High Enough" vs Gramatik "Stairway to Hip Hop Heaven" (remix of Led Zepplin "Stairway to Heaven")
Fissunix - "21 Babas"
http://vimeo.com/10671566 - Green Day "21 Guns" vs The Who "Baba O'Riley"