DOSBox

DOSBox is an emulator designed to allow MS-DOS games to be played on modern operating systems. Doom, Heretic, Hexen and others will generally run under DOSBox, although a fast computer is required for emulation at a playable speed; recent versions have greatly improved the performance.

However, with the advent of Source ports, DOSBox has been rendered pointless for some gameplay of DOOM. In fact, some source ports were released before DOSBox even matured to the point where DOOM or DOOM II for DOS would run smoothly.