At the risk of sounding totally reckless, I was reading the Wikipedia article on extended vocal technique when it mentioned helium, which artificially raises the frequency of one's voice, and sulfur hexafluoride, which will lower it. I'm not planning on using it myself (this second anyway), but it sounds really neat -- are there any popular pieces which demand that a vocalist inhale some SF6?
(I realize that this is the most "college-kid" post ever)