What exactly are those openings in the bellies of many jetliners?
That's an air inlet -- technically a "Ram Air" inlet -- for the air conditioning packs.  There is a "left" and "right" pack, which supply air to the forward and aft part of the aircraft respectively (although either pack can supply enough air to keep the aircraft pressurized to its max altitude).  They use high temperature/high pressure bleed air from the engines or APU, but they also get airflow from the ambient air which is used to cool the bleed air.  The inlet noted in the question is where the ambient air enters, and there are exhaust vents for this air on the bottom of the aircraft.  THIS air isn't used for the pressurization per se (i.e. it doesn't enter the cabin), but just for cooling the air that will enter the cabin (which came from the compressor in the engines/APU, upstream of the combustion chamber).

On the ground, there are louvers that close to keep water and debris out of these ducts, and fans are used to pull in air in that configuration.  In-flight, the louvers open and airflow is generally sufficient to move the air through the system (although the fans still provide some assist).

While there is some amount of drag here, it's fairly minimal, and the ability to keep the cabin pressurized and cool is sufficiently important that the little bit of drag that's caused here is considered acceptable.

Close-up photo of the inlet door (Next-Gen 737):



Close-up photo of the inlet door (Classic 737):



Photo of exhaust louvers:

source page for all of the above (with technical discussion of the air conditioning &amp; pressurization system): http://www.b737.org.uk/airconditioning.htm

Even more technical description of the system -- text &amp; schematics but no photos