Xenon {wiki} is the heaviest non-radioactive noble gas. It affects your voice like sulphur hexafluoride (previously at Neatorama) and can get you high like nitrous oxide (laughing gas). Don't try this at home; just watch these guys breathe it. Link (embedded YouTube clip)
That was freakin hilarious.
Love it when their voice gets all deep b/c it's like they're talking in slowmo hehe