There are some amazing tools available for developers on the web, from pretty-printing your code to validating it, from colour palettes for visual impairments to tools for testing APIs. Here are some of my favourites.
This is the equivalent site to Stack Overflow for asking user experience questions (usability and accessibility). These include everything from usability testing to user interfaces, UI patterns, mobile UI, material design, and more. Again, you can learn from reading other people's answers, or contribute your own.
This is a library of design solutions for various elements of the user interface, such as forms, logging in, logging out, registration, menus, headers, footers, and the rest. It is very useful for seeing what layout and information architecture solutions others have come up with.
There are CSS generators for Box Shadow, Text Shadow, Border Radius, Gradients, Buttons, and layout. There are many different ones, so pick the one that you like.
HTML color palette
There are many HTML colour code sites out there. One of my favourites is color-hex, because it provides all the tints and shades of each HTML colour.
Paletton - The Color Scheme Designer
This tool shows you different colours side-by-side, and what they look like to people with colour-blindness.
The wonder that is FontAwesome has already been mentioned on this blog, but it is really a great tool for creating icons and buttons, and ensuring that you have consistent user interfaces throughout your applications.
A huge online database of Unicode characters, for when FontAwesome doesn't have the icon you need.
This is a browser extension for resizing your browser window to simulate different screen resolutions, and is an essential tool in responsive design. You can then create media queries to optimise how your web page or mobile app will display in different screen resolutions. It is available for most browsers.
This is a showcase of snippets of code for doing cool stuff, such as cutting-edge CSS, jQuery and Angular.
Places for testing your code. Quite often, users of Stack Overflow will post working examples and solutions on Plunker or jsFiddle.
regex101.com - Regular Expressions 101
Great for testing your PHP online in a neutral environment.
There are various JSON validation tools available, so pick the one you like best. A good validator can pretty-print your JSON for you, and highlight any errors in it.
This is a desktop app for sending requests to web services to test that they are behaving as you expected. It's a powerful GUI platform to make your API development faster & easier, from building API requests through testing, documentation and sharing. There's a free Postman App for Mac, Windows, Linux or Chrome.