SameBoy is a user friendly, powerful and open source Game Boy, Game Boy Color and Super Game Boy emulator for macOS, Windows and Unix-like platforms.
It is extremely accurate and includes a wide range of both powerful debugging features and user-facing features, making it ideal for both casual players and developers.
Of course, it also has every feature one would expect from an emulator - from save states to scaling filters.

Includes a thumbnailer for GNOME as well.