Sometimes, I like looking up the definition of classes or functions from libraries or tools I use to understand how they are actually implemented. Back in the day, this meant figuring out how to get the source package of that library, downloading it and then searching inside its source files. But now that almost all libraries share their code on Github (or similar services), it has become quite convenient to look up source code repositories. The recent addition of the ability to open a Github repository using VS Code running in the browser is the icing on the cake!
Here are the source code repositories that I most frequently consult: