for the psion
and for the palm OS operated PDA's
the liberty gameboy emulator, it will cost you though about 16 bucks.
LIBERTY GB EMULATOR
LIBTERY GAMEBOY EMULATOR REVIEW
A 100% free GB EMULATOR FOR PALM
To use these, you must have the Gameboy Rom image. I found about 600 of these on edonkey. They work flawlessly, aside from the sound problems when playing a few games.