Microsoft has been gradually embracing open source software and releasing some of its software as open source. Here is a list of some of the Microsoft-owned software that has been released under an open source license:
.NET Core: An open-source, cross-platform version of Microsoft's .NET Framework for building modern applications.
PowerShell: An open-source command-line shell and scripting language designed for task automation and configuration management.
Visual Studio Code: A lightweight, cross-platform code editor for writing modern web and cloud applications that is built on top of the Electron framework.
SQL Server: A relational database management system that is available on both Windows and Linux.
Windows Terminal: A new terminal application for Windows 10 that provides users with access to Command Prompt, PowerShell, and other command-line tools in a single interface.
.NET Runtime: A runtime environment that enables developers to run .NET applications on different platforms.
Azure Functions Runtime: A serverless compute engine that enables developers to build and run event-driven applications.
Windows Calculator: A calculator application that is available on Windows 10.
PowerToys: A set of utilities for power users to customize their Windows experience.
Note: This list is not exhaustive and there may be other Microsoft-owned software that has been released with an open source license.
While GitHub is not software owned by Microsoft, it is a web-based hosting service for software development projects that was acquired by Microsoft in 2018. GitHub itself is not open source software, but Microsoft has released some of its own software as open source on GitHub, including the ones mentioned in the previous answer. Additionally, Microsoft has open sourced several tools and frameworks that are used in conjunction with GitHub, such as the Git Virtual File System (GVFS) and the GitHub Extension for Visual Studio. Microsoft also offers a number of open source projects on GitHub related to artificial intelligence, machine learning, and other areas of research and development.