Sulfur hexafluoride, SF6, is a very cool compound. It's a ridiculously heavy gas - 146.06 g/mol, and at 6.164 grams per liter it's over 5 times as dense as air. And yet, incredibly, it's absolutely harmless, except at extremely high concentrations, and only then because it displaces all possible oxygen!
Because it's almost completely nonreactive, it's safe to inhale in small quantities, so long as you are able to get it *out* of your lungs and breath in the carbon dioxide needed to cause the body's breathing reflex. Adam Savage demonstrated it on Mythbusters:
Being an inert gas, it's commonly used for things like window filling and as a non-conducting dielectric in high-voltage transformers and switches. It's also used as a tracer gas is public areas like subways systems because it's harmless, but that practice is mostly banned because SF6 is the most potent known greenhouse gas - it's got a warning potential 22800 times that of carbon dioxide, though it's currently only at a concentration of 6.5 parts per trillion (ppt), about one fifty-millionth (1/50,000,000) the concentration of CO2.
Other odd uses include making ultrasound-visible bubbles in blood vessels, in the casting of magnesium, as a tracer in deep-sea gas exchange studies, and use of its plasma for etching printed circuit boards (PCBs).
One of its few reactions is with lithium; the US Navy uses the reaction to produce high-pressure steam to propel Mark 50 antisubmarine torpedoes to speeds of over 40 knots.
Related compounds include Tellurium hexafluoride and Selenium hexafluoride; the latter of which is also a dense gas - 3.25 times air density - but is poisonous.
One of the few problems with SF6 is that in the presence of an electrical arc, it can be turned into highly poisonous Disulfur decafluoride; S2F10, a chemical weapon considered for use in WWII, that is reportedly so dangerous that a single breath can kill within a day.