I was just looking at Zend Server I am a little confused, is it something you run on Apache or something you run INSTEAD of Apache for using PHP?
It's a PHP stack, maintained by Zend instead of by your distribution's maintainers. You run it with Apache and it replaces your regular PHP and PHP extensions. On Windows and Mac it also includes Apache and MySQL if I remember correctly.
There are two editions,the free 'community edition' and a commercial edition that is just called Zend Server.
Both come with a GUI for easier management of settings, such as the php.ini settings with additional tooltips and help, or enabling/disabling extensions or viewing logfiles.
The free version includes the following extensions: Zend Optimizer+ (a good PHP optimizer and bytecode cache), the Zend Data Cache (an set of API functions to cache data or output in memory), the Zend Debugger (a very useful remote debugger and profiler that you control from Zend Studio or Eclipse PDT), Zend Guard Loader (enables loading of encrypted PHP files) and Zend Java Bridge.
The commercial version adds: Job Queue (API and GUI tooffload execution of long-running PHP scripts; enhanced alternative for cronjobs), Code Tracing (root-cause analyses and recording of application execution, manually or triggered by conditions), Monitoring (reports and logs application problems and allows replay in Zend Studio), Page Caching (cache output based on various rules), Download Server (offload transfer of large files so your webserver doesn't have to deal with them).
Compare the versions here: http://www.zend.com/en/products/server/editions
At work we're using the commercial Zend Server on several webservers and while it's not cheap we feel it's worth it (make sure you talk down the price though). For non-commercial and personal use the community edition is a good choice. The Debugger is excellent, as well as the Optimizer. Also, the ease of installation and configuration across different linux distributions is nice to have.
Hope I didn't sound to much like a salesman, but I'm quite pleased with the current Zend Server 5.
It has Apache included and if wanted you can integrate it with IIS. In short: it's a(n overpriced) PHP application server.