IMPORTANT
This mod is still a big work-in-progress, do not use on worlds you actually care about.
Worlds created with Biome ID Extender will be incompatible and break without it installed.
Some biomes may display the wrong biome name in F3, this is a known bug. It won't affect the generation at all and is just a client-server desync for some reasons.
Biome ID Extender
Biome ID Extender is a minecraft mod for Minecraft Forge 1.7.10 that aims to extend the biome ID limit to allow for more than 256 biomes in the game.
Mods tested to work fine
Highlands
ExtraBiomesXL
Ridiculous World
Thaumcraft
ChromatiCraft
Biomes O' Plenty
Mods with issues
Let's hope this list will stay small.
Alternate Terrain Generation (mostly works, small issues with glass blocks generating in the sky)
Realistic Terrain Generation (mostly works, same issues as ATG as well as the 'Hell' biome generating in the overworld)
NEID: It has certain overlaping changes with this mod which may trigger a crash under some circumstances, use EndlessIDs instead
Optifine: Crashes
Fastcraft : Crashes
Possibly other mods that abused the 255 limit that we have yet to patch, or mods with issues with haven't noticed.
Mods that probably won't be supported
Let's hope this one will as well.
F.A.Q
Q: Is mod x (not listed above) supported? A: Try and see. There's a chance that no further compatibility patches are needed. If it crashes, you can suggest it and I'll see what I can do, but first make sure it's not in the "Unsupported" list.
Q: So how many IDs exactly does this mod allow for? A: The exact number is hard to remember, just think 50,000. That's more than enough.
If you really want to know the real number, you pretty much can set any ID below 65,536, which should be plenty.
Q: What about block/ item IDs? A: Check out EndlessIDs.
Q: What about potion IDs? A: Check out Extended Potions. It extends the potion limit to 4096.
Q: What about enchantment IDs? A: Check out Enchantment ID Extender. It extends the enchantment limit to 4096.
Q: What about entity/ dimension IDs? A: Forge provides a way to register unlimited entity/dimension IDs. If a particular mod is not using it, it's the fault of the mod author.
Q: When port to version x.x.x? A: For 1.12+, just use Just Enough IDs.
Q: Modpacks? A: Of course! No need to ask.
Q: There is a crash! / This mod doesn't work with X! / I found a bug! A: You can always report it and I'll try and take a look.

Extends biome ID limit, allowing over 256 biomes in the game.