During the development of Survival Industry, we had a new tweaking tool created. The
JavaScript Mod Loader, or JMOD for short, is a tweaking tool that allows extensive tweaking of a game, including creation, modification, and removal of blocks, items and fluids, including recipes, ore dictionary entries, materials (for tools and armor), properties, (ore) placement, and even chest loot.
In addition to the list of features for this mod (which is considerable), there's also the mod's nature: It uses the JavaScript engine built right into Java 8 (called Nashorn) to allow your scripts to be as simple (a list of tweaks) or as complex (including loops, conditionals, objects, functions, etc.) as you want. And since the interpreter's built right into Java itself, this mod will not take the same performance hit that tweak mods using their own custom scripting languages suffer.
Also, JMOD is the only tweaking mod I know of that will actually allow the tweaker to add recipes to RotaryCraft machines.
The best part is that the tweaks are then zipped up with a "mod.json" file and appropriate resources (textures, language, etc.), and the zipfile is renamed with a "jmod" extension. In essence, the tweaks become a self-contained mod that you just place in the "mods" folder. As long as JMOD is installed, all the .jmod files will be loaded, all created items will be assigned to the name of your custom jmod (just like an actual mod), and the Forge mods list will actually show your tweaks as a mod. So, instead of, for example, "jmod:custom_block", your custom item will be named "mypack:custom_block".
This means that any modpack author can make custom items that actually belong to the modpack itself, rather than a specific "custom items" mod.