http://eaccelerator.net/is a free open-source PHP accelerator, optimizer, encoder and dynamic content cache. It increases the performance of PHP scripts by caching them in their compiled state, so that the overhead of compiling is almost completely eliminated. It also optimizes scripts to speed up their execution. eAccelerator typically reduces server load and increases the speed of your PHP code by 1-10 times.

eAccelerator stores compiled PHP scripts in shared memory and executes code directly from it. It creates locks only for a short time, while searching for a compiled PHP script in the cache, so one script can be executed simultaneously by several engines. Files that can't fit in shared memory are cached on disk only.

another project
Turck MMCache for PHP
Zend Performance Suite
PHP Accelerator
Alternative PHP Cache
AfterBurner Cache

Links
Caching PHP Programs with PEAR
Script Caching with PHP