Having Drush installed server wide is handy, especially if you have several users logging on the server. Use the appropriate url for the tarball, you can find the latest release of drush at http://drupal.org/project/drush. The lines below assume you have sudo access. If you don't, please check the Drush documentation to learn how to install Drush. The commands below have been tested successfully on Debian, Ubuntu, CentOS and should work for most *NIX systems.
sudo wget --quiet -O - http://ftp.drupal.org/files/projects/drush-7.x-5.6.tar.gz | sudo tar -zxf - -C /usr/local/share
sudo ln -s /usr/local/share/drush/drush /usr/local/bin/drush
The last line is for Drush to auto download the required library. If it displays a list of drush command, it means drush was installed successfully!
Some machines might not have the right folders created, so either update the paths above to match your environment, or create them beforehand:
sudo mkdir /usr/local/share/
sudo mkdir /usr/local/bin/