Optimise your JS files with this online compiler which features different optimisation levels. Once you have compiled your file it should still be gzipped to make it even smaller.
Browser plugin for Drupal.org - it's particularly useful for Drupal contributors.
A browser plugin that captures screenshots, let you annotate them, and with a click uploads them to share with others.
Reduce the amount of CSS code you write and make it easier to manage things like browser compatibility issues.
An article about Googles latest PageSpeed modules which they claim help render pages up to 2x faster.
Some inspirational uses of CSS3 to create design details that don't necessarily require the use of images.
A huge list of free programming books, which actually covers more than just programming languages.
A good article with some handy advice about creating sites with responsive designs.
Easily generate CSS 3 syntax with cross browser support.
Yahoo tool to remove unnecessary bytes from image files with optimisation techniques specific to image format.
PageSpeed Insights analyzes the content of a web page, then generates suggestions to make that page faster.
A good post on improving response times on Drupal. This includes time taken to generate/serve requests, number of requests needed to display the page, size of requests and time it takes for browsers to render.
SEO has changed significantly in recent years and this is a great list of the latest SEO considerations for your Drupal site.
SimplyTest.me lets you instantly create demo Drupal sites. You can also configure your sandbox Drupal version, install modules or use an existing distribution.
A handy article listing the best 20 Drupal SEO modules, along with brief descriptions of each.