Ammonia, or NH3, is a mildly toxic gas (but not that bad - you use it in metabolism). Go down one, though, and you get more lavishly toxic.
Phosphine, or PH3, is much more toxic (go down another, to arsine, and you've got another toxic beastie). Phosphine is used in fumigation for this very reason.
Going down a group will reliably get you the bizzaro version of most chemicals. Methane, CH4, is an inert, flammable gas - silane, SiH4, isn't just flammable. It'll burn without any ignition source, going up in a puff of sand - thats right, not a whoosh of CO2 - SiO2 is the solid stuff sand and glass is made of.
Did you watch Breaking Bad t'other day as well?
SAND! Dude, more! I totally want a sand explosion! Instant playgrounds!
Phosphine gas is also a byproduct of meth labs and it can be detected by a fishy/garlicky smell. It causes end-organ damage, so first responders in meth lab explosions, beware!
(I don't know this from personal experience btw... read about it for a class recently)
Lineberry TW. Methamphetamine abuse: a perfect storm of complications. Mayo Clin Proc. 2006 Jan;81(1):77-84.