Zend

Zend are best known for their open source Zend Framework, on top of which PHP 5 applications can be built easily. Some of the essential features are Zend_Mail and Zend_Feed, on top of which some of the biggest CMS's are built.

They also build Zend Studio and Zend Server, with coupons available for both. They are also giving away free zend t-shirts, fantastic. There is an enterprise level version of Zend Server which can handle massive simultaneous traffic bursts.

The Zend Framework optimizes php interpreting through a complete analysis of common code executions, leading to a low level rewrite of many core functions.