ReactOS

ReactOS is a free and open-source operating system that aims to be binary-compatible with Windows, allowing users to run Windows applications and drivers without Windows. It is developed by a volunteer community and released under open licenses, primarily the GNU General Public License, with some components under other licenses.

ReactOS is not based on Linux or Unix. It reimplements the Windows NT architecture, providing its own kernel, a Win32-compatible user-mode API, and subsystems such as the graphical user interface, networking, and a driver model intended to work with Windows drivers. The project explicitly targets binary compatibility with Windows programs and device drivers from the Windows NT lineage, though compatibility is partial and dependent on available components.

The project began in the late 1990s as an effort to create a free, open-source Windows-like operating system. Development is community-driven, with public repositories, issue trackers, and regular development releases. It has no corporate sponsor and relies on volunteers for coding, testing, and documentation.

Licensing is largely under the GNU GPL with some components under other licenses. The project distributes source code and prebuilt images under these licenses and welcomes contributors from the open-source community.

Status and reception: As of the mid-2020s, ReactOS remains in active alpha development. It is not a drop-in replacement for Windows, and hardware support is limited. Most users run it in virtual machines for testing or educational purposes, and compatibility with Windows software and drivers is partial and varies by program and version.