This file is linked to 
'How to install ionCube Loader & Zend Optimizer for Encoded PHP Files' here:
http://mewbies.com/how_to_install_ioncube_loader_and_zend__optimizer_for_encoded_php_files.htm

If you don't want to go through the hassle of registering to aquire the 
download link here are the latest direct links as of 21 Jun '13. For 
newer versions than listed; test it by editing the URL with the latest 
version's numbers.

Linux (64 bit) PHP 5.4 v6.0.0: 
http://downloads.zend.com/guard/6.0.0/ZendGuardLoader-70429-PHP-5.4-linux-glibc23-x86_64.tar.gz
Linux (32 bit) PHP 5.4 v6.0.0:  
http://downloads.zend.com/guard/6.0.0/ZendGuardLoader-70429-PHP-5.4-linux-glibc23-i386.tar.gz
Linux (64 bit) PHP 5.3 v5.5.0:
http://downloads.zend.com/guard/5.5.0/ZendGuardLoader-php-5.3-linux-glibc23-x86_64.tar.gz
Linux (32 bit) PHP 5.3 v5.5.0:
http://downloads.zend.com/guard/5.5.0/ZendGuardLoader-php-5.3-linux-glibc23-i386.tar.gz 

Windows        PHP 5.4 v6.0.0:
http://downloads.zend.com/guard/6.0.0/ZendGuardLoader-70429-PHP-5.4-Windows-x86.zip
Windows        PHP 5.3 v5.5.0:
http://downloads.zend.com/guard/5.5.0/ZendGuardLoader-php-5.3-Windows.zip

Mac OS X       PHP 5.4 v6.0.0:
http://downloads.zend.com/guard/6.0.0/ZendGuardLoader-70429-PHP-5.4-darwin10.7-x86_64.tar.gz
Mac OS X       PHP 5.3 v5.5.0:
http://downloads.zend.com/guard/5.5.0/ZendGuardLoader-php-5.3-darwin-i386.tar.gz
 

EXAMPLE:
I had installed Linux 32bit version 3.3.3 this is the link:
http://downloads.zend.com/optimizer/3.3.3/ZendOptimizer-3.3.3-linux-glibc23-i386.tar.gz
Then later I needed v3.3.9, so edited link and it worked:
http://downloads.zend.com/optimizer/3.3.9/ZendOptimizer-3.3.9-linux-glibc23-i386.tar.gz
Upper/lower casing must be correct e.g. z is not the same as Z