VisualBoy Advance emulates Gameboy Advance games for the Mac, and since the GBA also played Gameboy Color roms VisualBoy Advance does too. However, there is no support for original Gameboy games.
Like most emulators of consoles, VisualBoy Advance offers users some control over window size, sound and emulation speed. However, it does not offer controller support, although you can configure the keyboard to your liking. VisualBoy Advance also supports battery saves and save states, each of which have a handy folder.
Also, VisualBoy Advance supports GBA BIOS, but you don't need one to use it.
Running VisualBoy Advance I found a few problems with the sound (it skipped in places) and changing the screen size (the program crashed when I tried to do it while running a rom), but otherwise everything worked okay.