DOS and Windows

DOSBox
An open source DOS emulator for BeOS, Linux, Mac OS X, OS/2, and Windows. Primarily focuses on running DOS Games.
http://www.dosbox.com/

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.
http://dosemu.sourceforge.net/

Flopper
Open source bootable-disk emulator - it reads a diskette image file (a file that includes all the sectors of a floppy disk) and boots it. This way, users can boot and run a bootable diskette off of their hard drive.
http://www.oldskool.org/pc/flopper/

MacWindows
Solutions for integrating Macintosh and Windows.
http://www.macwindows.com/