The Virtual GameBoy is portable. Its distribution includes C source code which can be compiled on any computer (given that there are system-dependent drivers for this computer) and a set of sample drivers for Unix/XWindows. There are also versions of VGB compiled for MSWindows, Macintosh, and MSDOS. For more information, refer to documentation at above-mentioned WWW.
Another GameBoy emulator, called GBSim, is written by Jens Restemeier. It includes a very nice debugger, and is probably faster than VGB, but it only runs on MSDOS-running PCs, and has quite a few incompatibilities with the real thing.
Yvan Rivard is working on yet another emulator, PCBoy. It is also an MSDOS-only project, and it still can't run most of the games. PCBoy is written in 80x86 assembly language.
There is also a GameBoy-look-alike emulator for the Commodore Amiga called ToyBoy. This emulator can be obtained via FTP from WUARCHIVE or any other AmiNet site.
Gameboy68000.lha from WUARCHIVE
Unfortunately, this package is incomplete and HAS NOTHING TO DO with the real GameBoy. It was stolen from a UK-based company called Argonaut which does hardware and software development for Nintendo. According to Argonaut's managing director Jez San, ToyBoy was an in-house emulator written to see how difficult it would be to program GameBoy games. Since its authors knew nothing about internals of the real GameBoy when they were writing the emulator, ToyBoy is completely incompatible with GameBoy.