可以下载，是zend的强劲对手。是免费的。我已经在用啦。效果还不错。
修改 php.ini 加上：
zend_extension = /usr/local/lib/php_accelerator_1.3.3r2.so
phpa = on
phpa.tweaks = on
phpa.cache_dir = /tmp
phpa.file_perms = 400
phpa.cache_file_prune_period = 1h
phpa.cache_file_ttl = 1d
phpa.shm_size = 10
phpa.shm_key = 0xc0deb00
phpa.shm_perms = 666
phpa.shm_user = nobody
phpa.shm_group = nobody
phpa.shm_release_at_exit = on
phpa.shm_stats_check_period = 5m
phpa.shm_ttl = 12h
phpa.shm_max_processes = 512
phpa.shm_lock_threshold0 = 10

apachectl graceful 就能用啦。具体参数看它的文档。