Language: 

Spanish.  Argentineans speak a distinctive dialect of Spanish (we say "Castellano" - Castillian - as opposed to "Español"), more pronouned in Buenos Aires province than in other regions of Argentina.  For us geeks out there, Wikipedia has an interesting article on the characteristics and origins of the "castellano" spoken in Buenos Aires.  Not to worry; if you already speak some Spanish, Argentineans will understand you and you will understand them. 

Time Zone:

GMT -3.  With daylight savings time, it's an hour earlier. Argentina's time zone is one or two hours ahead of New York, two or three ahead of Chicago and four or five ahead of Los Angeles (depending on daylight savings).  It can be confusing because when we switch to daylight savings in the US, Argentina may or may not have switched to standard time (remember:  the seasons are reversed south of the equator!).  To add to the confusion, not all provinces outside of Buenos Aires are conforming to daylight savings.  Daylight savings time ends on midnight between Saturday, March 14, 2009 and Sunday, March 15, 2009 local daylight time and begins on midnight between Saturday, October 17, 2009 and Sunday, October 18, 2009 local standard time.  For more detail, see Time and Date.com.  A plus for most US visitors - the time zone is close to yours, so you'll experience little to no jet lag!

Currency:  

Peso.  The currency symbol used in Argentina for pesos is $.  When stating currency in US dollars, it will explicitly say "dolares" or sometimes "US$".  Also, a comma is used instead of a decimal point and vice versa, so $20,00 means 20.00 pesos and $20.000 means twenty thousand pesos.

   Exchange rate:

As of March 30, 2009, $3.72 pesos for US$1, $4.89 pesos for €1.  Yahoo Finance provides a handy currency converter.

Country telephone code:  +54

International Airport: 

Ezeiza (full name:  Aeropuerto Internacional de Ezeiza and also known as Ministro Pistarini International Airport) 

Airport Code:  EZE  

Airport web site: http://www.aa2000.com.ar/

Average temperatures:  

Argentina is in the southern hemisphere, with seasons the reverse of those in the northen hemisphere.  December through March are summer months and winter takes place from June to September. 

The average summer temperature in Buenos Aires is 77 F (24° C) and 50F (10°C) is average in the winter. 

In the Patagonia region (Bariloche, specifically), average temperatures go from 43F (6°C) in the winter, to 65F (18°C) in the summer. 

The Argentina Government site has temps for other major cities.

Electrical power: 

Careful!  Argentina uses the common European standard for power - 220 volt, 50-cycle alternating current.
Your phone, laptop and camera chargers may be compatible (check to see if they are marked as 100V-240V 50/60Hz). 
Check to see if your hair dryer also works (you may have to switch a setting).

Unlike the standard outlets in the US, power outlets in Argentina have 2 cylindrical holes (for non-grounded outlets) or 2 flat holes with a ground connection, so invest in a couple of power adapters.  Tip:  You can buy a $25+ kit with multiple adapters if you've decided to become a world traveler.  If not, you can get a single adapter for $4 at Walkabout Travel or other travel accessory stores.  

For any appliance that does not work with 220v, 50Hz, you will need a transformer/converter (or leave it at home - do you really need to bring your coffee maker?).  You can find them at Brookstone,  Walkabout Travel or other retailers.

The Tourism Office also has a comprehensive, downloadable guide.

Where in the world is Argentina?  Here's a handy map.