Same as yours +

• https://wandbox.org : an alternative to God bolt, when just needing to quickly test something (i.e. not needing the disassembly and multiple options of godbolt, just see if it compiles, or behave like expected on several compilers - I use that very often when writing code review comments to be sure not to advise rubbish...)
• https://regex101.com/ : not specifically for c++ but useful as well

Some great blogs too:

• https://www.fluentcpp.com/ : unfortunately not maintained anymore but many great articles mainly on readability
• https://www.cppstories.com/p/start-here/
• https://www.modernescpp.com/index.php
• https://arne-mertz.de/ : simplify c++

One must see YouTube video: https://youtu.be/2olsGf6JIkU

Perhaps some others I'll look tomorrow on my work computer.

The https://hackingcpp.com/cpp/cheat_sheets.html cheat sheets for std::algorithms and containers always save me a lot of time.

To plug shamelessly:

• https://github.com/jeaye/stdman - cppreference docs as man pages
• https://raw.githubusercontent.com/jeaye/value-category-cheatsheet/master/value-category-cheatsheet.pdf - A cheat sheet on C++14 value categories and common mistakes

One more I forgot is https://cppinsights.io : this online tool takes C++ code and output C++ code, the goal being to make the "magic" of the compiler visible (for example for(auto vi : std::vector...) is expended to iterators and the tool make it visible). It can help sometime when struggling with a difficult to understand issue.

Compiler explorer

I wish cppreference had better search. I usually end up using it through google/ddg to find what I really need. For example, searching chunk_view gives no results.