“XCache is a fast, stable PHP opcode cacher that has been tested and is now running on production servers under high load. It is tested (on linux) and supported on all of the latest PHP cvs branches such as PHP_4_3 PHP_4_4 PHP_5_0 PHP_5_1 PHP_5_2 HEAD(6.x). ThreadSafe/Windows is also supported. It overcomes a lot of problems that has been with other competing opcachers such as being able to be used with new PHP versions.”
Have been playing with it locally and used it on a customer install. Have been trying most of the opcode cachers over the past months, this one is the only cacher that is actively developed, apart from APC.