Most of the good rom hacks are GBA hacks of FIRE RED OR LEAF GREEN OR EMERALD!
the majority being FIRE RED!
to make them you need to apply the IPS patch to the rom
you should be able to play using a 3 in 1 loading them with GBA EXPLODER but you may need to also apply a battery fix with GBATA gba rom tool as all pokemon games need a REAL TIME CLOCK and afaik the 3 in 1 has problems with these roms
not all hacks work and those that do are BUGGY and GLITCHY an freeze and are generally messed up
theres loads of pokemon hacks out there
Pokemon Brown
Pokemon Forest Nature
Pokemon Gold Hoax
Pokemon Ice P cont.
Pokemon IceBlue
Pokemon Iolite
Pokemon Naranja (Orange)
Pokemon Prism
Pokemon Quartz P cont.
Pokemon Ruby-Shinou Edition
Pokemon Shiny Gold
Pokemon South Park Hoenn Stories
Pokemon Steel
Pokemon Chaos Black
and recent one Pokemon Liquid Ocean Alpha 1.0
some have even made it onto EBAY (Pokemon Chaos black with a printed box bodged together instructions and printed sticker on a cart that says NINTONDO on the back!!!! )
I've tried just about all of them and they all pretty much suck due to the crashes glitches etc!!
(Don't ask me where to get them YOU KNOW THE RULES!!!)
and DON'T ASK ME HOW TO MAKE OR PATCH THEM!!! theres guides out there on the net!!