eAccelerator PHP Accelerator

http://www.leigeber.com/2008/04/eaccelerator-php-accelerator/

Introduction to web caching on Leigeber (great web dev blog)

http://www.leigeber.com/2008/04/introduction-to-web-caching/

"Minify" your external JavaScript and CSS with PHP

http://www.thedanglybits.com/2007/06/21/minify-your-external-javascript-and-css-with-php/

also

http://code.google.com/p/minify/

RRDtool - Open source graphing for PHP and other languages

RRDtool is the OpenSource industry standard, high performance data logging and graphing system for time series data. Use it to write your custom monitoring shell scripts or create whole applications using its Perl, Python, Ruby, TCL or PHP bindings.

http://oss.oetiker.ch/rrdtool/

The most powerful MySQL command (Explain)

http://www.rawseo.com/news/2009/03/26/the-most-powerful-mysql-command/

Blocking dodgy referrers and blocking specific IPs with PHP

Here ya go:
http://perishablepress.com/press/2009/04/21/4g-ultimate-referrer-blacklist/
http://perishablepress.com/press/2009/06/08/block-multiple-ip-addresses-with-php/