Wine
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Clone OS to Run Windows Apps
An Australian developer is building software that runs Windows NT applications but doesn't require software from Microsoft.
DOSBox
An open source DOS emulator for BeOS, Linux, MacOS X, and Windows. Primarily focuses on running DOS Games.
DOSEMU
An emulator of the Microsoft Disk Operating System (MS-DOS) for Linux/X11. Enables Linux to run many DOS programs - including some DPMI applications.
Eliel Software Development
Software for developers and game players, featuring DodGE: the DOS Game Emulator for Windows.
Flopper
A bootable-disk emulator - it reads a diskette image file (a file that contains all the sectors of a floppy disk) and boots it. This way, you can boot (and run) a bootable diskette off of your hard drive.
Project David
A Windows compatibility middleware, which will enable all major Microsoft Windows applications to run on Linux. Describes the project.