Emulators

DOSBox - is a long gone from Windows–the command prompt inside Windows isn’t truly DOS anymore. Many old games simply won’t run from the Windows command prompt. They need true blue DOS. You can get the free DOSBox, which does a great job of emulating it. Install and run the program, then run any DOS game–or DOS application, for that matter–inside it. When you run DOSBox, it automatically sets any sound-system related variables, so that your sound system will work properly with your old games.
QEMU - is a generic and open source machine emulator and virtualizer using dynamic translation to achieve good emulation speed.
Virtualbox - is a family of powerful x86 virtualization products for enterprise as well as home use.
Wine - is an Open Source implementation of the Windows API on top of X, OpenGL, and Unix. It lets you run Windows programs on x86-based Unix systems. Operating system: Linux/Unix, OS X, Solaris.
ZSNES - is a Super Nintendo emulator programmed by zsKnight and _Demo_. It lets you play your Super Nintendo games on your PC. Some games work better than others, and it is very much a working progress. Operating system: Windows, Linux, FreeBSD, and DOS.