PHP Accelerators ,xcache and apc seems to be quite fast

eaccelerator is very good too (just installed on main production site and response from serve ~ 2x faster )

http://itst.net/wp-content/uploads/2006/10/PHP%20Bytecode%20Cacher%20Review.html
via
PHP on Fire: Five Opcode Caches compared.
http://itst.net/654-php-on-fire-three-opcode-caches-compared

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