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There are actually two of them, both stark contrasts of one another. They may not be some of the best ever made, but... they surprisingly sorta go well with the series. The second one has some language, but... some of the best songs do :P (Also, it's not letting me embed the vids tonight, so links you must get)
http://www.youtube.com/watch?v=mUSIHVyi75A
http://www.youtube.com/watch?v=HIvLX_xf8J8&NR=1