Simple Creator

This mod is called Simple Creator, not because it's simple to use, but because it was/is a simple mod to write.

It allows adding blocks and items by placing JSON's in a folder Structure under a SimplyCreated folder in the Minecraft directory.

It's JSON format is just a translation of what is possible in code without writing any custom code. (and some things are still missing)

Yes, many things you can do don't make sense, some might even break your game.

Usage:

To add blocks, place JSON's under:
minecraft/SimplyCreated/[your_modId]/blocks/[block_id].json
An example file can be seen here.
This file contains all used properties.
A list of all supported properties and explanations for some of them can be found here.
You can leave out any property, and a sensible default should be used instead.

To add items, place JSON's under:
minecraft/SimplyCreated/[your_modId]/items/[item_id].json
An example file can be seen here.
This file contains all used properties.
You can leave out any property, and a sensible default should be used instead.

For both, blocks and items, you will need to add models, block states, textures, loot tables, add them to tags to set mining levels and effective tools, and probably more via data/resource packs. I recommend using a mod like Global Packs in addition.

Disclaimer:

I am not responsible if this mod or a block/item created with it breaks your game. This mod is intended to be used by mod pack devs to e.g. add currencies or simple unique blocks.