Out of the box, Game Boy Advance flash cartridges let you play GBA games – not old Game Boy games.
Check out Goomba! It removes that annoying limitation by emulating a GB on the GBA.
You might recall that the GBA has GB emulation hardware in it, so this might not seem like much of a breakthrough. Not so!
GBA carts run at a different voltage than GB carts, so having a GBA flash cart running at old-school GB voltage levels wouldn’t work. This emulator truly emulates an old school Game Boy on the souped-up GBA hardware at full speed.
I just ran across it a few minutes ago, so I’ll check it out tonight!