In one of my previous blog posts I talked about using APC as an opcode cacher to speed up your site.
I thought it was worth mentioning a second time. This greatly improves the performance.
You can read more about the different opcode cachers and how to configure them here:
http://blog.crazytje.be/php-opcode-caching-why-use-an-opcode-cacher/
Part 4 will talk about optimizing your php code with a profiler(XDebug), expect more soon 
Optimizing your web server: Part 1 – Gzip
Optimizing your web server: Part 2 – Keep Alives
Optimizing your web server: Part 3 – Opcode Caching
Optimizing your web server: Part 4a – PHP
Optimizing your web server: Part 4b – XDebug Profiler
Optimizing your web server: Part 5 – CDN