ScummVM is a collection of game engine recreations. Originally designed to play LucasArts adventure games that use the SCUMM system (the VM in the name stands for virtual machine), it now also supports a variety of non-SCUMM games by companies like Revolution Software and Adventure Soft.
DOSBox is emulator software that emulates an IBM PC compatible computer running the older operating system, MS-DOS. Many IBM PC compatible graphics and sound cards are also emulated.
Boxer, based on DosBox, plays all the MS-DOS games on Mac OS X. Just drag-drop the games onto Boxer, and it will be playing in minutes.