www.draw.io
Professional, open source diagramming. Online or desktop, no registration.
www.deepl.com
Use the free DeepL Translator to translate your texts with the best machine translation available, powered by DeepL’s world-leading neural network technology. Currently supported languages are English, German, French, Spanish, Portuguese, Italian, Dutch, Polish, and Russian
valgrind
Valgrind is an instrumentation framework for building dynamic analysis tools. There are Valgrind tools that can automatically detect many memory management and threading bugs, and profile your programs in detail. You can also use Valgrind to build new tools.
software.nasa.gov
NASA Software Catalog offers hundreds of new software programs you can download for free to use in a wide variety of technical applications.
quick-bench.com
Quick-benchmark is a micro benchmarking tool intended to quickly and simply compare the performance of two or more code snippets.
miro.com
Online whiteboard
https://www.onlinegdb.com/online_c++_compiler
online compiler and debugger for c/c++
http://demangler.com/
GCC and MSVC C++ Demangler
graphsketch.com
Online draw math functions
godbolt.org
Compiler Explorer is an interactive tool that lets you type code in one window and see the results of its compilation in another window. Using the site should be pretty self-explanatory: by default the left hand pane is the source window and the right hand has the assembly output.