Master System and Game Gear

Emukon
A freeware emulator for Windows.
http://emukon.kontechs.de/

Massage
Freeware emulator for DOS.
http://www.enterspace.org/world/

MasterGear
Commercial emulator for Win32 and freeware emulator for FreeBSD, Linux, MacOS, OS/2, OSF/1, Solaris, SunOS and WinCE.
http://fms.komkon.org/MG/

MesaDX
A freeware emulator for Windows.
http://home.comcast.net/~mesadx/

Osmose
Open source emulator for Linux and Windows.
http://bcz.emu-france.com/

Past-O-Rama
Freeware emulator for Win32.
http://www.system16.com/pastorama/

Semus
Open source emulator for Windows.
http://jiggawatt.org/badc0de/semus/

SMaSher
A freeware emulator for Windows.
http://www.eidolons-inn.net/tiki-index.php?page=Triton

SMS Power
Offers current news on ROM releases as well as downloads.
http://smspower.speedhost.com/

vbsms+
Open source emulator for Windows.
http://olaf.efewe.com/vbsms+.php