Just gathering frontend resources as sites and some useful apps for beginner developers, Never mind sending a pull request to add some new resources.
👉 For regular expressions: https://regexr.com/ | https://regex101.com/
👉 To generate SVG background: https://haikei.app/
👉 HTML cheat sheet quick revise: https://htmlcheatsheet.com/#
👉 React cheat sheet: https://devhints.io/react
👉 All programming cheat sheet: https://cheatography.com/
👉 Frontend helper site: https://codepen.io/
👉 Your software Quora: https://stackoverflow.com/
👉 Find free images here: https://unsplash.com/ | https://www.pexels.com/ | https://freesvg.org/
👉 Get fonts from here: https://fonts.google.com
👉 Color helper site: https://colorhunt.co/
👉 https://randomuser.me/api (Generate random users on every call)
👉 https://robohash.org/ (add your name and get random alien pictures)
👉 https://fakestoreapi.com/ (Fake products with image URL and description)
👉 https://jsonplaceholder.typicode.com/ (Multiple Things you can fetch)
👉 https://jsonplaceholder.ir/users (Again fake user with profile pictures)
👉 https://placeholder.com/ (Get image placeholder of any size)