https://make.wordpress.org/core/handbook/

WordPres Core Contributor Handbook

http://wp-a2z.org/

Dynamic API Reference for WP core, RestAPI, WooCommerce, etc

http://v2.wp-api.org/

WordPress Rest API documentation

https://woothemes.github.io/woocommerce-rest-api-docs/

Woocommerce Rest API documentation

https://businessbloomer.com/woocommerce-tips/visual-hook-series/

BusinessBloomer.com Visual WooCommerce Hooks Resource

https://chrome.google.com/webstore/detail/lightshot-screenshot-tool/mbniclmhobmnbdlbpiphghaielnnpgdp?hl=en

Lightshot (screenshot tool) – A simple chrome extension for taking screenshots with arrows, added explanatory text, etc. A really great communication tool.

https://chrome.google.com/webstore/detail/advanced-rest-client/hgmloofddffdnphfgcellkdfbfbjeloo

Advanced Rest Client – Chrome extension for the RestAPI.

https://chrome.google.com/webstore/detail/builtwith-technology-prof/dapjbgnjinbpoindlpdmhochffioedbn?hl=en

BuiltWith – Chrome extension for determining various aspects of a site from: hosting, frameworks, languages, libraries, content encoding or other technologies used to make the site.

https://chrome.google.com/webstore/detail/eye-dropper/hmdcmlfkchdmnmnmheododdhjedfccka/related?hl=en

Eye Dropper Color Picker – Select colors from any page to get the hex, rgb or name values of any given color.

http://sourceforge.net/projects/pacmanager/

PAC is a Perl/GTK replacement for SecureCRT/Putty/etc (linux ssh/telnet/… gui)… It provides a GUI to configure connections: users, passwords, EXPECT regular expressions, macros, etc.

https://poedit.net/wordpress

Code Localisation / Translation

http://lighttable.com/

Light Table – The next generation code editor. Sponsored in part by Matt Mullenweg.

https://atom.io/

Atom – An advanced text and code editor.

http://www.sublimetext.com/

Sublime Text – An advanced text and code editor.

http://wpgear.org/

WordPress development tools

https://tell.wtf/

Font development tools

https://generatewp.com/

WordPress code generation tools

http://wpbin.io/

WordPress-centric code pastebin tool

http://wphierarchy.com/

Visual interactive reference to the WordPress template hierarchy

https://infinitewp.com/

Manage multiple WordPress sites from one control panel

https://mainwp.com/

Manage multiple WordPress sites from one control panel

TOP 3 tools for WordPress users:

https://extendsclass.com/php-tester.html

PHP code checker

https://extendsclass.com/css-generator.html

CSS Generator

https://extendsclass.com/csv-generator.html

Random CSV/SQL Generator

http://vim.rtorr.com/

Vim Cheatsheet

http://mod-rewrite-cheatsheet.com/

Apache httpd mod_rewrite cheatsheet

https://www.regex101.com/

Regular Expression (Regex) sandbox

https://docs.vagrantup.com/v2/getting-started/project_setup.html

Vagrant Setup, virtual disposable servers for local installation. Setup default operating/development environments for yourself in just a few minutes.

http://jsfiddle.net

Live collaborative interactive code fiddle

http://codepen.io/

Code playground and sharing

http://cssanimate.com/

CSS animations code generator

http://schema.org/docs/gs.html

Getting Started with Schema

https://search.google.com/structured-data/testing-tool

Google Schema Testing Tool