Web Design and Development Tools:

Selenium.dev - automates browsers. That's it! (download page) (download server) JAVA Required
Postman (download page) (download link x64)
Scout [sass] (download page) (download link)
Emscripten - a complete compiler toolchain to WebAssembly, using LLVM, with a special focus on speed, size, and the Web platform
JSFiddle - free, easy-to-use, HTML, JS, & CSS testing platform
Brackets - A modern, open-source text editor that understands web design with live preview
Codepen.io - CodePen is a social development environment for front-end designers and developers
JSHint - free Static Code Analysis Tool for JavaScript (download page)
htmx.org - htmx allows you to access AJAX, WebSockets and Server-Sent Events directly in HTML, using attributes, so you can build modern user interfaces with the simplicity and power of hypertext. htmx is small (~7k min.gz'd), dependency-free, extendable & IE11 compatible
uptimerobot.com - Downtime Happens. Get Notified!
Pingler your URL for FREE - Drive SEO Search Traffic to your site
JSON-LD Schema Generator For SEO

- Visual primitives for the component age. Use the best bits of ES6 and CSS to style your apps without stress
Scrapy, a fast high-level web crawling & scraping framework for Python. https://scrapy.org
Scrapyd is a service for running Scrapy spiders.
SpiderKeeper - admin UI for scrapy/open-source scrapinghub
scraper_ai – Extract data from any website, detect changes
Scraper API - Proxy API for Web Scraping in 2020
cloudflare.com - DDoS protection and traffic acceleration for all your network infrastructure

Miscellaneous Web Tools:

httrack (download page) (download link x64) - Download web sites as a copy of HTML files. Doesn't work with JS.
ʕ•ᴥ•ʔ Bear - Free, no-nonsense, super-fast blogging - No javascript, no stylesheets, no trackers. Just your words.
Archive.org - historical data on web site changes
Google Images - search all images on the internet using the Googles search algorithm
Smithsonian open-access Image Collection - search 120M Smithsonian images
QR Code Generator from the ZXing Project
Sharp - an automatic API generation library for Python Flask and JavaScript
donotlink.it - Whenever you link a website, its position in search engines is strengthened. Sometimes you want to post a link to a website without improving its rank. This is where donotlink.it comes in.
PrivacyPolicy.com - ready to use website privacy policies
WebMaster Tool Kit - (download page) Color Picker & Measurement Tool
namecheckr.com - Check domain & social username availability across multiple networks
jsonapi.co - JSON Real-Time Editor
IconSear.ch - search of 50K royalty-free SVG icons
URL Fuzzer - Discover hidden files and directories
grep.app - Search across a half-million git repositories - cross-search of git repos across the internet
Text Fragments - Create a link to specific text in somebody else's page or pdf (Chrome experimental) (Draft Community Group Report)

https://example.org/file.pdf#page=2&search=findme

Web Based Accessibility checkers:

The Functional Accessibility Evaluator (FAE) evaluates a website or a single web page based on the W3C Web Content Accessibility Guidelines (WCAG) 2.0 Level A and AA requirements.
WAVE Web Accessibility Evaluation Tool - WAVE is a suite of evaluation tools that helps authors make their web content more accessible to individuals with disabilities.

Mockups/WireFrames

Mockup Tiger (download page) (download link)
moqups (download page)
balsamiq.com - With Balsamiq Wireframes, anyone can design great websites.
Placeholdifier - Turn any website into a live wireframe
storybook.js.org - Build bulletproof UI components faster. Storybook is an open source tool for developing UI components in isolation for React, Vue, Angular, and more

CSS Tools:

Google Fonts
Fonts101.com - (free) Turn handwriting into a font
glyphter.com - (paid) Turn SVG into font
Howtocenterincss.com - centering in CSS
CSS3 Generator - Randy Jensen • Lorem Ipsum Generator • Unshorten URLs
CSS3 Button.net - This one offers the ability to add multiple text shadows, inner-shadows, border, and font colors whereas the last one really didn’t get into that much detail
Showing File Types in Links with CSS
Tracking users via CSS
Here’s what I didn’t know about “content”

Browser Extensions for Web Developers:

Simple Link Checker (Mozilla) - Redirect Link Checker will check all links on your page to find broken and redirect links
cssbuilder (chrome) - adds easy to use CSS3 design widget to Chrome
WordPress Theme Detector and Plugins Detector (chrome) - This tool detects theme and plugins used on WordPress sites and displays information about them
Web Sniffer (chrome) - View all HTTP Requests and Responses sent between the Web browser and the Web server
Wappalyzer (chrome) - a utility that uncovers the technologies used on websites
WhatRuns (chrome) - Discover what runs a website. Frameworks, Analytics Tools, WordPress Plugins, Fonts - you name it
WhatFont (chrome) - the easiest way to find out the fonts used in a webpage

Web Frameworks:
flask - python
laravel - PHP
Cordova - Mobile apps with HTML, CSS & JS

JavaScript Libraries & Frameworks:
roughnotation.com - A small JavaScript library to create and animate annotations on a web page
form-validation.js
Elevator.js - fixes those awkward "scroll to top" moments the old fashioned way
vuejs.org - The Progressive JavaScript Framework
vuetifyjs - a Vue UI Library with beautifully handcrafted Material Components
reactjs - A JavaScript library for building user interfaces
material-ui - React components for faster and easier web development
angular.io - Develop Across All Platforms
emberjs - a productive, battle-tested JavaScript framework for building modern web applications
nodejs.org - a JavaScript runtime built on Chrome's V8 JavaScript engine
gatsbyjs - a free and open-source framework based on React that helps developers build blazing-fast websites and apps
Nextjs.org - React Framework
Livewire - a full-stack framework for Laravel that makes building dynamic interfaces simple, without leaving the comfort of Laravel
svelte.dev - js framework - runs at build time, converting your components into highly efficient imperative code that surgically updates the DOM
Google News JS - an open-source package for scraping Google News data. It provides easy API access to main Google News features

CSS Frameworks:
XP.css - a framework to look like WinXP or Win98
mono-color (download) - accessible HTML/CSS charts with markdown support
ChartSS.css - a framework to provide flat mono or color controls
newcss.net - basic, vanilla, classless CSS framework for simple sites of the only HTML
w3.css - a modern CSS framework with support for desktop, tablet, and mobile design by default, is smaller and faster than similar CSS frameworks, was designed to be a high-quality alternative to Bootstrap, was designed to be smaller, faster, easier to learn, and easier to use, was also designed to be independent of jQuery or any other JavaScript library
Bootstrap - the world’s most popular front-end open-source toolkit, featuring Sass variables and mixins, a responsive grid system, extensive prebuilt components, and powerful JavaScript plugins
Foundation - The most advanced responsive front-end framework in the world
bulma.io - a free, open-source CSS framework based on Flexbox and used by more than 200,000 developers
skeleton - A dead simple, responsive boilerplate
purecss.io - A set of small, responsive CSS modules that you can use in every web project
groundworkcss - A Responsive HTML5, CSS and Javascript Framework
cardinalcss - Cardinal is a modular, “mobile-first” CSS framework built with performance and scalability in mind
powertocss - A versatile CSS Framework. Simple, light, and responsive
muellergridsystem - a modular grid system for responsive/adaptive and non–responsive layouts, based on Compass
bootflat - an open source Flat UI KIT based on Bootstrap 3.3.0 CSS framework
Pattern.css - CSS only library to fill the empty background with beautiful patterns