The Master System (originally known as the Sega Mark III in Japan) is a 3rd generation console created by Sega. It was a direct rival to the NES and despite being a failure in Japan and the US, it was a success in Europe and Brazil.
MEKA emulates the Master System and its handheld counterpart, the Game Gear as well as support for the SG-1000 and the ColecoVision. It has the highest compatibility rate being able to run most of the library including obscure Korean games that on other emulators might not run and is able to use various controllers such as the Light Phaser as well as the FM sound unit for better sound for games that support it. It also features a tile viewer, a palette viewer and a memory editor making it easier to rip from games.
Another emulator that emulates the Master System and the Game Gear with similar functions (graphic viewer, map viewer, and palette viewer) to MEKA. It can directly save currently loaded graphics as an image file under a dedicated "Tiles" folder, ontop of the common quick screenshot feature.
Emulicious is a Master System, Game Gear and a Game Boy emulator that runs on Java (however it doesn't require it to be installed since there is an optional version with the binaries included). It is newer than MEKA so it is more compatible with newer computers and has more accurate sound with less issues. It can run most Master System games. Like MEKA it has a tile viewer, a palette viewer and a memory editor so again easier to rip games however the palettes can not be edited and the tiles are double sized. On the other hand, no forced PNGs are outputted when print screened so more useful for computers with SSD drives and can be set to native size no matter what resolution your screen is.