Sulfur hexafluoride is a gas that's several times heavier than air. When you inhale it, it leads to the exact opposite of helium voice: a deep, rumbling vocalization that sounds suspiciously like a record on slow speed.
You can read the explanation behind sulfur hexafluoride's effect on your voice here. But for now, here's Neil Patrick Harris with a couple lungs full of the heavy stuff. (Kelly Ripa was there, too. But we all know you're here for NPH and his Buffalo Bill impressions.)
H/T Rebecca Guenard