Alternative PHP Cache, better known as APC, is a free download that can be installed by using the PECL installer.
When using Linux, simply pass this command...
pecl install apc
After apc installs, the apc.so extension will need to be added to the server's php.ini.
extension="apc.so"
Once added, restart apache.
To be sure that apc installed correctly, simply fire off a phpinfo() and check its output.