NineGB is a port of the Nintendo Gameboy emulator shipped with the Plan9Front operating system (a fork of Plan 9 from Bell Labs) to other operating systems, started by me.
The emulator's not very accurate and the sound emulation is disabled because it's currently bad.
Anyway, I have ported it to Visopsys as well, and it works quite well except that sometimes the game screen can lose focus, and keystrokes won't be registered - and the solution for the time being is clicking the game screen.
The loss of focus happens for example when you navigate the menus by pressing Alt, without choosing anything, and you close the menu by pressing Alt again - nothing in the window seems to have focus anymore (most probably a bug).
Screenshot:

Download it here.
The binary for Visopsys is included and named `ninegb_vsys', simply by clicking on it in the file manager will prompt you for a Gameboy ROM.
Enjoy.