If you have a palm or a, now useless because of the ultra cool PSP, Zodiac. I feel your pain. This is the best app for palm EVER. It's called Little John by Yoyofr, a NES, SNES, GB, NEOGEO and Sega Emulator all in one. Sadly no GBA but still really great. I've noticed high rom compatibility. Unlike Firestorm GBA emulator that has zero rom compatibility, worst emulator ever.
Get the Zodiac version here.
Get the palm version here.
And find some really neat projects on his main page here.