I strongly disagree with using web sites for tools that can be installed on your own computer. And in a professional environment, you should not send your source code to anyone, and you should be capable of working without an internet connection. It's even more obvious with CIs (not sending code, and packaging builds without being connected).

For example:

• Replace DevDocs with Zeal on Windows, or Dash on macOS. Those tools saved my ass a few times when I had a bad internet connection. It's always up-to-date and you can select what you want to have locally.
• Replace Code Beautify by a command-line tool or library that can automatically format your code. There is always one that is good or official for every language (pep8, Black, rust-fmt, clang-format, etc.) Its usage should be transparent and invisible, and the configuration can be set in stone only once for all the developers of your team. It can be called automatically when your save a file, commit (as a hook), or to allow the CI to check that your code is formatted properly.

My rule for writing code like a pro is to automate everything you can, and be independent. It's obvious for command-line tools like formatters or linters, but it's also very important for third-party dependencies that can be cached or saved on a local server like JFrog. This way, you will never be taken hostage by a site that stops being free, that is down, or that you can't access due to a network issue. Last but not least, it's a great way to start thinking about the automation of your build, and maybe later move on to creating reproducible builds.

(sorry for the rant)

Theses are some interesting sites. I'll use some of them for sure.