Development can be hard work, and there are a lot of tools that can make your life easy in the development world.

Here I list some tools that I use to achieve web development.

Regexr
This site lets you build regex expressions while explaining how it will behave. Supports JavaScript & PHP/PCRE RegEx. Very useful if dealing with complex expressions.
regexr.com

Built With
This page can explore other sites and describe which technologies or tools were implemented.
builtwith.com

Dummy JSON
This page provides you a lot of data in JSON format, very useful to mock up different types of data.
dummyjson.com

Metatags
This tools let you explore the metadata of a site. Can be used to enhance SEO and Open Graph information.
metatags.io

Facebook debugger web tool
Similar to metatags, this tools explore metadata information from any website.
developers.facebook.com/tools/debug

Photopea
This amazing tool lets you work with graphics. Its functionality is similar to Adobe Photoshop
www.photopea.com

Google Analytics
With this platform, you can create a Google tag or use the Analytics SDK to integrate to your website, track events and the type of your visitors. Useful to create reports and analyze util data.
https://analytics.google.com

Free Image Stocks
These websites let you get free and high-quality images.
unsplash.com
pexels.com
rawpixel.com
pond5.com

Freepik
Freepik offers a widely amount of editable files free.
freepik.com

Icon stocks
In these sites, you will find cool icon collections to use in your websites.
iconfinder.com
icons8.com
flaticon.com

Code Sandbox
In this site, you can connect with your github account and serve repositories online with other people.
https://codesandbox.io/

Deployment sites
This list makes reference to some popular sites that provide needed infrastructure where you can deploy your projects. These sites have free and paid plans.
Supabase
Heroku
Firebase
Vercel
railway.app

Request Bin
This site lets you mock up endpoints, so you can test HTTP calls.
requestbin.com

Image space Mockup
Do you need to mock an image fast without creating the graphic? Try this page. You only need to specify the width and the height at the end of the url.
Image of 1000 x 200

Tailwind Colors
Have a fast reference of the colors available in the Tailwind lib.
tailwindcss.com

JSON Viewer (Chrome Extension)
Have a pretty visualization of JSON files when they are open in your explorer.
github.com/tulios/json-viewer

What are some other cool tools or sites that should be mentioned?

I read you in the comments...