ReactOS
ReactOS is a free and open-source operating system designed to be binary-compatible with Microsoft Windows applications and drivers. It aims to replicate the Windows NT architecture, allowing users to run Windows software without requiring a Microsoft license. The project focuses on providing a familiar Windows-like environment while being built from scratch with clean-room reverse engineering.
Developers should learn ReactOS for exploring Windows-compatible OS internals, contributing to open-source OS development, or testing cross-platform applications in a Windows-like environment without licensing costs. It's particularly useful for those interested in operating system design, legacy Windows software support, or educational purposes in system programming.