Sometimes you will see a two letter country code at the end of the URL instead of the three letter organization code. Most URLs in the United States do not use the .us country code. Some other country codes are:

.au - Australia
.br - Brazil
.ca - Canada
.ch - Switzerland
.mx - Mexico
.nz - New Zealand
.cl - Chile
.de - Germany
.eg - Egypt
.jp - Japan
.nl - Netherlands
.uk - United Kingdom