MySQLTuner : MySQLTuner is a script written in Perl that will assist you with your MySQL configuration and make recommendations for increased performance and stabi...
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...
Beekeeper Studio : Open Source cross-platform SQL Editor and Database Manager. Use Beekeeper Studio to query and manage your relational databases, like MySQL, Postgres, ...
Dex : Dex is a MongoDB performance tuning tool that compares queries to the available indexes in the queried collection(s) and generates index suggestions b...
Sitespeed.io : Sitespeed.io is an open source tool that helps you analyze and optimize your website speed and performance based on performance best practices. It col...
Vitess is a database clustering system for horizontal scaling of MySQL.
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...
Minimize round-trip times : An article on the best practices to apply in order to minimize HTTP round-trip times
SitePoint : SitePoint provides cutting-edge content for web professionals — developers, designers, programmers, freelancers and site owners.
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
Nice write-up on techniques for scaling up a web application.