Resources

Site and Code Auditors

Linters

JSLint
ESLint

Validators

HTML5 Structural Validator
W3C CSS Validation Service
Online JSON validator

Minifiers & Beautifiers

Google Closure Compiler (for JavaScript)
Kangax HTML minifier
JSON minifier
CSS Formatter
TinyPNG

Formal Auditors

PageSpeed Insights
Lighthouse
GTMetrix
Dareboost
Pingdom
WebPageTest
WebHint

Compression/Speed Enabled Tests

GiftOfSpeed (Gzip test)
IPVOID Brotli Test
KeyCDN HTTP/2 Test

Site Security

CSP evaluator
Security Headers
HTTP HTTPS Header Check (view HTTP request and response headers)
Mozilla Observatory

Accessibility

WAVE Accessibility Tool (online version)
WAVE Accessibility Tool (Browser Extensions)
WAVE Text and Background Color-contrast Checker
AXE Color-contrast Checker (Online version)
AXE Chrome Extension (more than just color-contrast analysis)
Automatic Color Generator for Text, Provided User-specified Background Color
W3C Aria Landmarks Primer
W3C Accessibility Evaluation Tools List
Sitepoint Accessibility Checker (Chrome Extension)
Evaluera (WCAG 2.0 Tester)

Semantics | Schema

Rich Results in Google Powered by Schema
A Primer on Schema
Xoocode (Adavanced Schema Markup Examples)
Structured Data Markup Helper (Requires Google Sign-in)
Schema Markup Validator
Google Structured Data Examples: Article (with links to other schema types)
Yandex Structured Data Testing Tool
Rich Results Preview
Online Keyword Extractor

Favicons & Rich Links

Social Share Preview
Favicon Checker
Facebook Metadata Checker
Favicon Generator
LinkedIn Post Inspector
Twitter Card Validator

SEO

Technical SEO Tools
SEO SiteCheckUp
SEOptimer
Nibbler
SEMrush

Odds and Ends

Google Fonts Downloader
Meta Tags
Vendor Prefix Generator
CSS Specificity Calculator
Status Codes, Response Headers, and Redirect Chains Reporter
404 Checker
Sitemap Generator
Web Hypertext Application Technology Working Group (WHATWG)