"eAccelerator is a free open-source PHP accelerator, optimizer, 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."
In Debian stable unfortunately you wont find it apt-get'able but you can use the debianize script from dotdeb to create a .deb file for you from the source code.
root:~#./debianize eaccelerator-0.9.5.1 Creating php5-eaccelerator... PHP version : 5.2.0-8+etch4 PHP Api : 20041225 Configuring for: PHP Api Version: 20041225 Zend Module Api No: 20060613 Zend Extension Api No: 220060519 ....
dpkg-deb: building package `php5-eaccelerator' in `../php5-eaccelerator_5.2.0-8+etch4_i386.deb'.
and then install it via dpkg -i php5-eaccelerator_5.2.0-8+etch4_i386.deb