Boxer is a DOS game emulator for OS X, built around the powerful DOSBox. Boxer aims to make it easy and painless to play your DOS games.
BSNES has a somewhat different purpose to most emulators; it focuses on accuracy over performance. To that end, it does not include any game specific hacks, or idle-loop skipping optimizations commonly found in other emulators. To add to the fun, it uses a cycle accurate hardware emulation.
The Darwine project intends to port and develop WINE as well as other supporting tools that will allow Darwin and Mac OS X users to run Windows Applications, and to provide a Win32 API compatibility at application source code level.
DOSBOX is a DOS-emulator that uses the SDL-library which makes DOSBox very easy to port to different platforms. DOSBox also emulates CPU:286/386 realmode/protected mode, directory filesystem/XMS/EMS, Tandy/Hercues/CGA/EGA/VGA/VESA graphics, and a SoundBlaster/Gravis sound card.
Parallels Desktop for Mac
(commercial) Parallels Desktop for Mac is powerful desktop virtualization solution that provides the ability to develop, test, and deploy in multiple operating systems on a single PC. It supports nearly every standard x86-based operating system, including the entire Windows family, multiple Linux distributions, Solaris, OS/2, MS-DOS, and FreeBSD, each of which can be launched and utilized in completely networked, fully portable, totally independent virtual containers. Since guest operating systems are self-contained and directly access hardware profiles, users never need to partition hard drives, allocate system resources, or continually reboot the host operating system.