Weird Al has certainly written a fair number of great nerdy rap parodies, like It's All About the Pentiums. I don't see that they're particularly Pathfinderish, though.
Several people have already pointed out another song parody about D&D, called Roll a d6, on these boards.
Six months ago, there was a thread on the subject, in which someone introduced me to MC Frontalot, who raps about a lot of nerdy subjects, including D&D (although he doesn't do parodies), but the coolest one was the rap named in that thread, It Is Pitch Dark, about Zork.
But of all the songs listed in that thread, the one that truly captured D&D / Pathfinder the best, in my opinion, wasn't a rap at all. It was Never Split the Party.