OS & Compatibility Layers (Desktop Software Reimplementations): open-source projects that exist because someone reverse engineered a closed product, sorted by GitHub stars.
Valve's Wine-based compatibility tool for Steam Play
Android app running Windows x86 apps via Wine and Box86/Box64
Vulkan-based implementation of Direct3D 8/9/10/11 for Wine and Linux
Free Windows-compatible operating system reimplementing NT
Community Proton build with extra Wine/media patches
macOS GUI wrapper for running Windows apps with Wine
Darwin/macOS translation layer for Linux
Compatibility layer that runs Windows applications on Unix by reimplementing Win32
High-level emulator for early iPhone OS apps
Wrapper tool to port Windows software to macOS using Wine
Runs 16-bit Windows 3.x programs on 64-bit Windows using Wine code
Accuracy-focused reimplementation of Microsoft XNA 4.0
Direct3D 12 implementation on Vulkan used by Proton
J2ME Loader, a J2ME emulator for Android
Open source reimplementation of BeOS
Wrapper fixing compatibility of old DirectX/DirectDraw games on modern Windows
PlayStation Vita to Horizon OS (Nintendo Switch) translation layer
iOS 16 userland and XNU kernel bring-up on the Redmi Note 10 Pro (SM7150)
MS-DOS compatible operating system kernel
Free implementation of the OpenStep/Cocoa Foundation API
Runs Microsoft NTVDM (16-bit DOS subsystem) on 64-bit Windows
iOS emulator for Windows that runs iOS apps via translated binaries
Re-implementation of PalmOS that runs Palm m68k apps on modern systems
Sound Blaster emulation for DOS on modern HDA and AC97 sound hardware
Reimplementation of the Aero Glass blur effect for Windows 10 and 11 DWM
FreeJ2ME, a free J2ME emulator with libretro, AWT and SDL2 frontends
Accuracy-focused XAudio reimplementation for open platforms
Runs 32- and 64-bit Windows games on a stock iPhone with a controller
Open source reimplementation of the AmigaOS 3.1 APIs
Reverse-engineered source code of GEOS 2.0 for the Commodore 64/128
Repo hosting Advanced Mac Substitute, a reimplementation of classic Mac OS APIs
Emulator of IBM System/370, ESA/390 and z/Architecture mainframes
SquirrelJME, a clean-room Java ME 8 virtual machine
Compatibility layer that runs Macintosh Programmer's Workshop tools on modern systems
Nintendo Switch Horizon/NX kernel reimplementation
Z80 emulator plus CP/M-80 BDOS reimplementation to run CP/M software on macOS
Shim that lets Huawei eNSP run on VirtualBox 7.x without downgrading
Sound Blaster emulation for DOS on HDA hardware
Modern fork of Executor, a classic Mac OS reimplementation
User-mode NT and Linux emulator to run Windows and Linux programs across platforms
Emulator that reimplements The MajorBBS/Worldgroup APIs to run its BBS modules
Open source OS/2 clone (user-mode parts)
XInput reimplementation compatible with DirectInput controllers
Runs Windows 98 apps and games in the browser via x86 and Win32 emulation
Windows 7 AuthUI recreated via ConsoleLogon reimplementation
Open-source reimplementation of Apple's XPC library
Hercules (spinhawk), an IBM System/370, ESA/390 and z/Architecture mainframe emulator
Reimplementation of XInput and other gamepad APIs over SDL3
Runs Mac OS X 10.6 Snow Leopard applications on other systems
Faithful reimplementation of the Windows cmd.exe interpreter
Partial re-implementation of Microsoft's DirectMusic adaptive soundtrack API
Reimplementation of the Nintendo Switch firmware
MicroEmu, a pure Java implementation of Java ME
Reverse engineering the undocumented Windows UI library dui70.dll
OS/2 compatibility layer for running OS/2 binaries on Linux
Open-source reimplementation of the Nintendo Switch Horizon microkernel
Calling the undocumented Windows Component Based Servicing (CBS) API
Open-source reimplementation of the Microsoft WebView2Loader
Decompilation of Windows 7's Desktop Window Manager (dwm.exe and uDWM.dll)
Reverse-engineered C++ Aureal 3D audio API interface from 1999
Wine extension that installs and runs signed MSIX Windows Store apps on Linux
Installer and AppImage that run Shapr3D on Linux via wine-uwp
Android gaming environment for Apple Silicon Macs with a native Metal player
Translation layer to run Android apps natively on Linux by reimplementing Android APIs
Generic reimplementation of the INtime RTOS APIs
Loads Windows NDIS Wi-Fi drivers in the Linux kernel
WineHQ Direct3D 12 to Vulkan translation library
Experimental patch set on top of Wine