SMS+/MacOS is a Sega Master System and Sega Game Gear emulator for the Macintosh.
The hardware for the Sega Master System and Game Gear consoles is almost identical from a technical standpoint, which means that when you emulate one, it is trivial to emulate the other. SMS Plus features accurate emulation of both systems, with full sound support. Just like most of the other emulators on this site, it can use GZ compressed ROMs transparently.
 
SMS Plus was developed originally by Charles MacDonald. The source code to the original version is available at his web site.