Oddball compound

Unusual transition element oxides

Transparent volatile liquid metal oxides (/rusts). Note that those are highly toxic.

• http://en.wikipedia.org/wiki/Osmium_tetroxide
• http://en.wikipedia.org/wiki/Ruthenium_tetroxide
• http://en.wikipedia.org/wiki/Rhenium(VII)_oxide

Rather inert compounds with fluorine

• Sulfur hexafluoride
• Nitrogen trifluoride

Other compounds with unusual properties

• Carbon disulfide
• Trimethylphosphine (leave to Wikipedia - please come back again)
• Carbon suboxide (leave to Wikipedia - please come back again)

Carbon suboxide has a low energy state in earth’s oxidative environment and can be polymerized to a solid that could easily be stored by today’s means. When chemomechanical converters will become available there most likely will be better storage methods for depleted energy available though. So its just a curiosity. Note: Somewhat unintuitively the compound C₂O₂ (ethylene dione) is very unstable. It has a short lifetime even at low temperatures. This is one of the more subtel instances where one can see that the "periodic table as construction kit" metaphor must often be taken with a grain of salt.

Compounds dominantly containing nitrogen

Those are usually quite unstable to explosive.

• http://en.wikipedia.org/wiki/Nitrous_oxide
• http://en.wikipedia.org/wiki/Ammonium_azide
• http://en.wikipedia.org/wiki/Tetrazene
• http://en.wikipedia.org/wiki/Ammonium_nitrate
• http://en.wikipedia.org/wiki/Ammonium_nitrite

Others

Grey α-tin: Tin has a fully nonmetallic form that takes on the more sparse crystal structure of silicon due to covalent bond coordination. Replacing some silicon atoms in a silicon quartz or silicon carbide crystal with tin is likely to lead to stable structures with maybe desirable properties. https://en.wikipedia.org/wiki/Tin_pest

Xenon dioxide: Under high pressure xenon supposedly can replace silicon in quartz. It is believed that the theoretically predicted amount of Xenon that is missing in our atmosphere is trapped this way inside the earth. https://en.wikipedia.org/wiki/Xenon_dioxide Highly localized pressure (strained structures) can make (single or multiple substitutions of silicon with xenon) stable at macroscopic ambient pressures. Xenon is not too abundant so applications (whatever they are) are not likely to become widespread.