Visual Studio Code
Visual Studio Code (also known shorthand as VSCode) is an Integrated development environment (IDE) developed and maintained by Microsoft. It boasts of built in syntax highlighting, workspaces, Windows Powershell integration, themes, easy-to-install extensions, IntelliSense, debugging, and ability to preview most webpage and image extensions. Being one of the most popular code editors, Visual Studio has been in development for almost 10 years, and is Open Source on Github. Visual Studio Code is very useful and quite popular among Source mappers and modders, as many extensions exist to ease the editing and creation of many Valve formats.
Uses with Valve's works
VS Code's extension marketplace has many extensions catered to Source Engine specific files. VS Code can edit any text-based Source Engine file, including VMT, VDF, QC, Vscript, and VMF files.