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.
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.
dosemu.sourceforge.net/

Flopper - A 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, you can boot (and run) a bootable diskette off of your hard drive.
www.oldskool.org/pc/flopper/

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