We've talked about vent-free and electric fireplaces, which are a great solution when you need a fireplace in an area that doesn't have good access to the building envelope.  But there's another way, the Power Vent.  A typical venting system has limitations, it's basically just operating on air pressure and good old physics.  You need a certain amount of rise for a given run, to make sure that there's enough draft to pull the exhaust out of the home, which in turn creates a vacuum that pulls in replacement air from outside.  Practically, this means that your run is limited by the rise, for example if you can only go up 5', you may be able to go over 10', it varies by fireplace model.  It also means that you are limited in how many elbows you can put in the pipe, typically 270 degrees is your max.  So if your fireplace is below grade, but there's a bedroom directly above, or you're on an interior wall and 30' away from the outside wall, your gas fireplace just may not work.

Enter the power vent.  It's just a fan really, which helps to suck that exhaust through and increase the power of the vacuum to pull in fresh air.  This extends your maximum vent run to roughly 100', even horizontally.  It also means that you can go down into a crawl space and out, although once you go down you typically can't come back up.  Finally it allows you to put a bunch of bends in the run, often 6-10 ninety degree elbows instead of 3.  So if you have to twist around some HVAC in the attic, or maneuver to get over to a pre-existing chase that you want to re-use, the power vent is your way. 

You'll have to check with the dealer of the fireplace you want to see which options you have, but there are basically three types of power vent:  Inline, End of Line Horizontal, and End of Line Vertical.  The Inline power vent goes somewhere inside the home.  This is usually going to be in an attic or a crawl space, sometimes in a closet.  You'll need to be able to access it for service, and you'll want to make sure it's far enough away from the fireplace, and in a relatively sound-proof space so that you don't hear the hum.  The end of line horizontal comes out the wall and sits outside the house.  It's pretty common, and several manufacturers even offer a flush-mount one that sits nice and flat on the outside of the wall.  Otherwise they're pretty bulky, so you won't want it in a high visibility area.  The end of line vertical will come up through the roof or a chase.  The upside is that it can be hidden behind a decorative shroud, the downside is that you have to get up on the roof to access it, and it does need support so coming straight out the roof often doesn't work very well.

A big thing to keep in mind is that your fireplace's control module will be talking with the power vent.  There's a pressure switch that will make sure the fan is functioning before the burner is allowed to turn on.  This means that if you don't have power your fan won't work, and consequently neither will your fireplace.  Battery backup systems won't cut it for this application, so get yourself a generator. 

As a general rule, avoid power vents if you can, ask your local dealer to come out and do an assessment to see if there's a way you can gravity vent that fireplace.  But if it's what you have to do to stay cozy, then it's what you have to do.