DOSBox: Free, open-source DOS emulation
Running games during the DOS era was trouble enough, and today exploring those old games can be a nightmare. Sound card issues, memory management and other peculiarities of early PC gaming amplified as OS technology progressed and passed these titles by, leaving many of them orphaned and unable to run on modern systems. Today's answer to this problem isn't digging up an old system or rooting around for special drivers, it's emulation: free and open-source DOSBox.
DOSbox uses the SDL library to emulate the early operating environment of the PC. It supports the classic file systems, CPUs and sound and videocards of that era almost flawlessly, with hundreds of titles on the compatibility list. DOSBox has become something of a standby in this regard, with popular websites like GOG.com using it extensively to get their catalog of PC golden oldies running perfectly for modern customers. SDL also means versions exist for many platforms, including Linux and OS X.