[Xorlev]

-APC cache. I recommend APC cache over eAccelerator if you're not on a shared environment, or need the decoding abilities of Zend Optimizer. If you do, then run eAccelerator (keep an eye out for bugs) + Zend Optimizer.

-Run PHP 5.1.2+. PHP 5 is horribly slow, and I prefer it over PHP 4. By default, PHP 5 also passes objects as references rather than copies which can save on RAM.

-PHP via FastCGI or similar. I run my PHP with Litespeed webserver...it has a custom SAPI module that's faster than FastCGI, but even so FastCGI is far faster than CGI or mod_php.

-Dump Apache. Apache is a great server, but it tries to be everything to everyone and is rather clunky. Use Lighttpd (can use FastCGI), or give Litespeed webserver a try.

-Setup MySQL with a query cache. It takes more RAM, but it saves the need for executing the same SELECT queries over and over when the dataset is still the same.

-Disable per-thread and per-forum "who's online". It's a neat feature, and I use it, but if you're squeezing for performance this can help.

-Plugin Based Template Cache. When used in "multiple files" mode, this works to great effect with APC.

I have some more I'll post later.