APC

APC : A free, open source PHP Accelerator available in PECL repositories.
This is going to be bundled with PHP 6.0, the next big version of PHP with full unicode support.
Actively developed as of October 2007.

eAccelerator

eAccelerator : A free, open source PHP Accelerator for Windows and Linux (started as a Turck MMcache fork in 2005).

If you are looking for a PHP accelerator, especially one that also works on Windows, check out eAccelerator: “eAccelerator is a further development from mmcache PHP Accelerator & Encoder. It increases performance of PHP scripts by caching them in compiled state, so that the overhead of compiling is almost completely eliminated.”
Actively developed as of October 2007.

Xcache

Xcache: A free, open source PHP Accelerator by one of the lighttpd programmers.
Actively developed as of October 2007.

Zend

Zend Performance suite

eZ publish run time performance increases with Zend Accelerator

eZ publish source code compilation with Zend Optimizer
Note: Zend Optimizer does not result in any performance increase on it's own since it's not caching the resulted PHP opcode.
Actively developed as of October 2007.

Turck MMCache

Turck MMCache : A free, open source PHP Accelerator.
Not actively developed since 2003, use eAccelerator instead.

The ionCube PHP Accelerator (phpa)

The ionCube PHP Accelerator
Not actively developed since 2005

References:

http://www.ez.no/community/blog/a_free_php_accelerator_for_win_and_lin

http://ez.no/ez_publish/documentation/configuration/optimization/php_acceleration