DirectX / SDL (external). Supports text mode, CGA, EGA, Hercules, and VGA programs.
Uses internal speaker
DOSBox is a free and open-source emulator designed for running DOS programs. It is a good choice for running many DOS programs on modern systems, since it can slow down games that run too fast on modern processors. DOSBox is available for many platforms, such as Windows, Mac OS X, and Linux. It has even been ported to the Sony PSP.