I wrote a small PHP caching system and I'm releasing it here under a BSD open source license.
SimpleCache is a file-based caching system that is used to cache dynamically generated content, as from a database and/or heavy processing. It's use is similar to the caching system that is part of the PHP Smarty templating system (except Smarty caches HTML output, whereas SimpleCache caches whatever kind of data you want to cache).
SimpleCache is very flexible in that you can cache as finely grained as you want, you can cache whatever data you want, is very easy to install (simply include one file into your PHP files) and is easy to insert into your code. It requires PHP 5.
It does not cache compiled intermediate PHP code, like Alternative PHP Cache does. It does not cache data to memory, only to files.
SimpleCache is here. Instructions on how to use it are in that file.