Context: PHP, programming languages, web servers
Zend (or more properly, Zend Technologies Limited) is the company devoted to providing commercial support to the PHP server-side scripting language. Their major product is the Zend Engine, which is the scripting engine behind PHP.
They also provide products that are generally useful for commercially-oriented PHP developers. These include:
Of course, all these things cost money, something that open source developers may not have. Then again, I don't think open source developers would require some of the Zend's products anyway.