Ask HN: What are your top 3 coolest software engineering tools?

Let's start a discussion about cool tools we use in software engineering that others might not know about, here's my top 3.

    1) BitBucket's CI/CD pipelines - For running various tests and checks on code when a commit is made. [1]
    2) CTest - Automated C/C++ testing. [2]
    3) Arduino-CLI - For Arduino testing. [3]

What's yours?

Links:

    1) https://cmake.org/cmake/help/book/masteringcmake/chapter/Testing%20With%20CMake%20and%20CTest.html
    2) https://bitbucket.org/product/features/pipelines.
    3) https://github.com/arduino/arduino-cli