DevDocs: Fast, organized, and searchable API documentation
HTTP Status Codes: An easy to reference database of HTTP Status Codes with their definitions and helpful code references all in one place
Repl.it: A quick online IDE for quick coding and collaboration
CodePen: An online editor for creating and sharing HTTP, CSS, and JS with a live preview
code2flow: Easy flowcharts expressed as code
CyberChef: "The Cyber Swiss Army Knife" that can parse and/or convert just about anything
JSBen.ch: Benchmark JavaScript
Animista: A collection of customizable CSS animations
Favicon Generator: Generate favicon-related assets for all platforms
Devhints: Developer cheatsheets
ColorSpace: Generate color pallets from a color
uiGradients: A ton of good looking gradients
Big list of malicious input: A big list of things that may break things
Big-O Cheat Sheet: Big-O algorithm complexity cheat sheet
Nginx Config: A tool that generates Nginx configs
Forward Email: A free, encrypted, and open-source email forwarding service for custom domains
Flaticon Pattern Generator: Simple pattern generator
Hero Patterns: Color-independent SVG background patterns
URL Checker: Useful for bypassing downloads behind ads
Regex 101: Very useful website for building and testing regular expressions
TypeSource: Font inspiration
Palettable: Explore color palettes.
A Soft Murmur: Relax
Pingly: Free hosted email on your domain
Sentry: Great error tracking
Netdata: Great performance and health monitoring for your servers and applications
Trello: Task tracking boards
RequestBin: Inspect web requests and more
QuickBMS Universal script based file extractor and reimporter. Supports tons of games and file formats, archives, encryptions, compressions, obfuscations, and other algorithms
DeepL Translator: A good machine translator for Japanese