MS-DOS (Microsoft Disk Operating System)
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.
Click to Download Version 0.7.4 (1.4MB)
Click to Download Version 0.7.4 (1.6MB)
(LAST UPDATED: May 12, 2010)
Official Site - Additional Files, Forum, Older Versions, Source Code, Related DOS Links, Compatibility Reports

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.
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)
Official Site - Links To Free Games, Beta Versions, Release Notes, Instructions