The PHP Accelerator (Cache). The Accelerator is a plugin extension to PHP that accelerates script execution typically between 2 and 3 times by caching compiled code for instant reuse the next time your scripts are accessed.
If you look at Nick's history, he says he started learning how to use the Zend engine only on the 14th of August 2001, and on the 20th he's got the first prototype cache working. A very impressive hack! [PHP Everywhere]