Use The Index, Luke! : A tutorial covering SQL indexing and SQL tuning for developers: avoiding unnecessary details about database internals.
SQL indexing is the most effect...
MySQLTuner : MySQLTuner is a script written in Perl that will assist you with your MySQL configuration and make recommendations for increased performance and stabi...
Nuster : Nuster is a high performance caching proxy server based on HAProxy. It is 100% compatible with HAProxy, and takes full advantage of the ACL functional...
Phinx : Phinx makes it ridiculously easy to manage the database migrations for your PHP app. In less than 5 minutes you can install Phinx and create your firs...
Mobile Perf bookmarklet : It displays a menu with links that load other bookmarklets including Firebug Lite, Page Resources, DOM Monster, SpriteMe, and CSSess. The Mobile Perf ...
Vitess is a database clustering system for horizontal scaling of MySQL.
Minimize round-trip times : An article on the best practices to apply in order to minimize HTTP round-trip times
Let's make the web faster : There are many ways to make websites run faster. In this section, you can discover performance best practices that real web professionals employ in th...
Diagnostic.css : Diagnostic.css is a stylesheet which allows the user to test for common errors in a page's markup.
Also : https://github.com/karlgroves/diagnostic.css
Squoosh : Squoosh is an image compression web app that allows you to dive into the advanced options provided by various image compressors.
OpenCalais : Calais is a rapidly growing toolkit of capabilities that allow you to readily incorporate state-of-the-art semantic functionality within your blog, co...
Nice write-up on techniques for scaling up a web application.