I'm looking for a portable air compressor to leave in my vehicle for emergencies.  There are 12v DC models made to plug directly into the cigarette lighter, or 120v AC models.

I have a DC converter that plugs into my cig lighter to give me a couple of outlets.  I hope this isn't a stupid question -- Is it safe to run something like an air compressor off of that converter? So far I've used it for charging small electronics, but nothing that pulls a lot of voltage.

No.  Most cig lighter sockets are rated at 10, maybe 15 amps max.  10 amps at 12 vdc = 1 amp at 120 vac less any converter losses.

Viair has a pretty good reputation among the 4x4 and pickup truck air horn crowds:  Viair 12 vdc compressor

Good luck.

Excellent! Thanks

There are also a variety of portable compressors that hook directly to your battery terminals.  Like this

I have one of the compressors from the link above.  It works and was cheap (used really only for airing up after beach trips).  It works, but its not fast.  More airflow = $$$ (look at the other viair models).

In some of the solar / wind energy forums they talk about 12v systems that if you have a 12v system and need to run anything of significant power the amp draw on your system is huge. The example he gave was a 120v 1500w space heater. That if you were using a 12v battery and inverter it would draw almost 100 amps at 12v DC to obtain the 120v and 1500w.

get a 12v that hooks straight to the battery.