Starboard and Larboard

The Vikings called the side of their ship its board, 

They placed the steering oar, the "star" on the right side of the ship, 

thus that side became known as the "star board." 

.... And, because the oar was in the right side, 

the ship was tied to the dock at the left side. 

This was known as the loading side or "larboard". 

Later, it was decided that "larboard" and "starboard" were too similar, especially when trying to be heard over the roar of a heavy sea, so the phrase became the "side at which you tied up to in port" or the "port" side.

from http://www.navy.mil/navydata/traditions/html/navyterm.html

this also explains why people who fall off ships go "overboard"

image from bbc.co.uk