I could upload a lot of emulators here, but since they are updated every once in a while, that would be pointless. I am instead linking Zophar where to find the emulators, check below.

Sega Master System / Game Gear emulators

Meka is the best emulator for DOS and Windows. If you have an old computer (like a Pentium), you might want to try Massage.

Sega Mega Drive emulators

For Windows, Kega and Gens are the best. For DOS, Genecyst and KGen98 are recommended.