devdocs.io

DevDocs brings together numerous API documentation in a single, searchable interface. You will find docs related to various programming languages and technologies in one place.

css-tricks.com
You can master your web development skills by learning everything about CSS from this website. If you didn't already know, CSS is what makes pages on the web look beautiful.

overapi.com
OverAPI is one of the most beautiful and useful websites for all developers. This website has cheat sheets for the majority of programming languages. Please take a look at it right now.