MS-DOS (Microsoft Disk Operating System)
 DOSBox
DOSBox is a must-have for classic DOS gaming. It has high compatibility for old games that do not run well on Windows. There is also a version for OS X as well, which is good for me because I don't use my PC these days.
DOWNLOADS
Click to Download Version 0.7.4 (1.4MB)
Click to Download Version 0.7.4 (1.6MB)
(LAST UPDATED: May 12, 2010)
LINKS
Official Site - Additional Files, Forum, Older Versions, Source Code, Related DOS Links, Compatibility Reports
SCREENSHOTS

 Boxer
Boxer is a slightly tweaked version of DOSBox that is better integrated with OS X. It allows you to create game shortcuts that can be loaded directly from the finder window, and contains many other useful features that I have not had a chance to explore.
DOWNLOADS
Click to Download Version 1.0 RC2 (18.1MB)
(LAST UPDATED: February 22, 2011)
Click to Download Version 0.8.7 (18.1MB)

(LAST UPDATED: August 30, 2009)
LINKS
Official Site - Links To Free Games, Beta Versions, Release Notes, Instructions
SCREENSHOTS