Interesting tidbit why a lot of the countries drive where they do:
In Britain and many such nations they were used to riding horses since medival times. Since most people are right handed, keeping to the left ensured that your sword was available for striking if need be. Also with a sword hanging off your left side (the easiest postition to draw from for a right handed person), riding on the left ensured that your scabbard wouldn't hit another rider as you passed.
In the US when our country formed swords were mostly gone. People were driving large wagons with teams of horses though. The early versions of this often had to driver seat, and instead the driver sat on the back rear horse. This allowed him to keep his right hand available for lashing the horses, and driving on the right side ensured that as he passed another wagon he could ensure that the wheels would clear each other.
These days though some countries have changed either by preference or by legislation. MOST seem to drive on the right side though.
For a breakdown on each country:
http://en.wikipedia.org/wiki/Right-_and_left-hand_traffic