KiGB is a Game Boy and Super Game Boy emulator for the Mac. It was made by Richard Bannister, who's also worked on Nestopia and other emulators for the Mac. Nestopia and KiGB are identical, with the exception that one emulates the NES and the other the Gameboy.
The emulator doesn't come with any games. You have to use your own ROM files. When you do load a ROM, you will notice that the game window is rather small. That is the original resolution of Gameboy games. You can either drag on the edge of the window to make it bigger, or press the "Escape" key on your keyboard to activate a full-screen mode.
The Preferences window is full of settings that you can change to modify the emulation process. You can change the speed of emulation, adjust the frame rate and size of some of the effects. You can also map the controls on your keyboard by dragging and dropping. If you have a joystick, you can also configure it.
I tested this emulator with a ROM of Battleship, one of the games that I played most on the GB. It ran perfectly. The music and sounds were great and the graphics were just as I remembered them. Most games should be supported by this emulator.