php-accelerator.co.uk has an accelerator which almost matches Zend's own one, and it's free! So yes, it's worth it (and they are good).
In case you want to know how it works, this is how. When a file is called (index.php) the server reads that file, parses it, fills in all the gaps, loads all the other files, gets all the variables sorted, etc etc etc, and then runs the PHP executable to execute the page and produce the output. The problem is that this cycle is repeated every time the page is called.
What the accelerator does is presume that the page hasn't changed it's coding in between requests, therefore doesn't need reloading and recompiling. It stores the (almost) last product, and executes that! Simple!