Host of this wiki!
Lots of knowledgeable FF4, FF5 and FF6 folks hang out there!
Mnrogar's Den (forum)
Registrations are closed but it's a good source of info about FF3us.
General ROM hacking community site that aims to bring several different areas of the ROM hacking community together. It has a large database of patches, utilities, documents and fonts.
Once was one of the central place of ROM hacking but the activity there has declined gradually.
Forum dedicated to difficulty hack on many systems. RotDS and Brave New world have a subforum there as well as other mods.
My Barren Webpage
Webpage of assassin, veteran hacker, that hosts many patches that are mostly bugfixes. You can also find on this site a commented C2 disassembly and other documents.
Imzogelmo's Not-Even-Penultimate ROM Resource
Here you can find patches and essential documents for event editing, such as the event dump and event commands list.
Good resource for disassembly docs (bank C0, C1 and C3 most importantly). They host a variety of patches, including their Pandora's Box hack.
ZED's Unoriginal White Sheet
Host of various hacking documents and original patches. There are also FF3us documents such as the Sketch bug guide.
Leet Sketcher's Patches for Final Fantasy III/VI
Website hosting various recent bugfix patches.
SilentEnigma's Cool Things
Site of a hacker that came out of the grave. Hosts a few patches.
Patches, utilities and documents like the enemy command script guide and commented C2 disassembly.
Drakkhen's Patch Pages
Host of some patches, utilities and a partial FFVI Advance ROM map.
Some patches, utilities and the incredible snes9x debugger.
Lord J's website
Tables, data formats and some utilities.
Hacks and utilities.
FF6 on Spriters Resource
Basically almost all the game graphics are there.
Video Games Sprites
PC, NPC, enemies and objects.
FF6 on vgmaps.com
FF6j English translation page.
RPG One FF6 Translation page (archive.org)
Mainly for archival purpose of the most famous FF6j translation.
Terminus Traduction FF6 translation
Most famous French translation.
Encyclopédie de FF6
French and English FF6 translation site. Last updated in 2005 and partially broken.
FF6 on DataCrystal
RHDN wiki entry on FF6.
Mnrogar's Den (website)
Some documents and nice savestates stack.
FF6 on FantasyAnime
Savestates and other stuff…
Neglected hacking website that is not updated often enough.
Return of the Dark Sorcerer: A FFVI mod
Home of RotDS.
FF6: The Eternal Crystals
Home page of FF6:EC.
Inu's Sandbox (Japanese)
Data analyzer for FF4A, FF5A, FF6A.
Japanese FF hacking page.