Zend Optimizer+ is the new opcode cache and optimizer that was open-sourced by Zend Technologies after already being part of their proprietary Zend Server software. Zend Optimizer+ optimizes the PHP bytecode and is able to store pre-compiled bytecode in shared memory. More information on the optimizer is covered by the PHP Wiki.
Also new to PHP 5.5 is an array_column() function, bug-fixes, support for changing the process title in CLI/CLI-Server SAPIs, support for non-scalar iterator keys in foreach loops, and much more.
More details are covered by the PHP 5.5 Beta 1 NEWS file. The next beta is expected in April of PHP 5.5.