got a big list on wikipedia of them http://en.wikipedia.org/wiki/List_of_PHP_accelerators
What are people using? is there a best? (and free?)
so I take it no one is useing them then. . . hmmm
I use php5-xcache and have never had any problems with it.
Note that the article you mentioned only mentions STABLE with xcache:
"XCache is a fast, stable PHP opcode cacher ..."
But, I might consider APC in the future, especially if it's included in PHP 6 Core.