Retro OS & Compatibility (Desktop Software Reimplementations): open-source projects that exist because someone reverse engineered a closed product, sorted by GitHub stars.
Compatibility layer that reimplements newer Windows APIs on Windows XP/2003
DirectDraw and Direct3D 1-7 compatibility layer for modern Windows
Web rendering proxy that lets vintage browsers view modern websites
Pseudo-driver that converts Direct3D 8 calls and shaders to Direct3D 9
Emulator that runs 32-bit Windows programs on other platforms via Wine
Thunks that provide missing newer Win32 APIs so programs run on Windows XP/Vista
Software emulation of the Roland MT-32 and related MIDI synths, from reverse-engineered ROMs
Emulation of the Roland SC-55 sound module from decapped chip and firmware RE
HTTP 1.x proxy that makes old web browsers usable on the modern web
DirectSound DLL replacement that restores EAX and 3D audio via OpenAL Soft
Runs DOS programs under Linux with a DOS environment emulator
ESP8266/ESP32 modem-to-internet emulator for retro computers
Emulator that runs Apple's NewtonOS on modern operating systems
Portable Z80 CP/M 2.2 emulator that reimplements the BDOS on modern hosts
Open-source AFP/AppleTalk file server for classic and modern Macs
Scratch-built open-source ROM BIOS for IBM PC, XT and 8088 clones
Search engine that strips the web down for vintage computers
Open-source reimplementation of Apple Macintosh System 7 for modern hardware
Tools including vamos, which runs AmigaOS programs by emulating the OS API
Disassembly of the Apple II Desktop ProDOS GUI
Palm OS emulator for the web and mobile
Free reimplementation of the OpenStep/Cocoa AppKit GUI library
Minimalist Macintosh 128K emulator
Clean-room open-source replacement ROMs for the Commodore 64 and other retro computers
Free replacement for the Atari ST TOS operating system ROM
Windows DLL that tunnels IPX/SPX over UDP so old LAN games work on modern Windows
DOS extender that runs Win32 console and some GUI programs under plain DOS
Highly accurate Yamaha OPL3 emulator based on die-shot reverse engineering
64-bit FreeDOS-derived DOS kernel used by dosemu2
FreeDOS command shell, a clean-room COMMAND.COM replacement
Extended free distribution of the Digital Research GEM GUI for DOS
Converts DirectInput 1-7 calls to DirectInput 8 for old game compatibility
Hayes modem emulator that connects retro computers to telnet BBSes
AT&T 3B1 / 7300 UNIX PC emulator
Source code for Apple II system software and DOS
Project to reverse engineer the Classic Mac OS
Universal Windows CE emulator (CE Runtime Foundation)
News site served in simple HTML for vintage computers
DOSBox IPX-over-UDP server with TAP bridge and Quake proxying for retro LAN play
Runs MS-DOS command-line programs on Win32/x64 by emulating DOS
Source code reconstruction of 86-DOS 0.11
Source of the original dgVoodoo 3dfx Glide wrapper
TypeScript implementation of the Palm HotSync protocol
Interface for Apple's LocalTalk networking protocol
AppleTalk router supporting LToUDP, TashTalk and EtherTalk
Tool to build and unpack classic Mac OS NewWorld ROM images
AppleTalk file and netboot server for Apple II computers
Tools for syncing with Palm OS devices on Unix-like systems
Tools for connecting to Psion EPOC devices over the PLP link protocol
Reimplementation of Microsoft's DirectPlay API for old multiplayer games
Wrapper that runs 3dfx Glide games on OpenGL
Disassembly of Altair (Microsoft) 680 BASIC for the Motorola 6800
CP/M system and software reverse engineering
x86 emulator with a DOS reimplementation for running old PC games
Disassembly of MS-DOS 1.25 programs released only as binaries, with rebuildable sources
Disassemblies and reverse engineering of MSX code
NEC PC-8001 and PC-8801 ROM reverse engineering
Reverse engineering of the PipeDream application for the Cambridge Z88
Reverse engineering of 8-bit Sharp MZ-series software
Reverse engineering of Sord computers
Disassembly of files from the s2-bits archive of early UNIX at The UNIX Heritage Society