LootTweaker is an addon for CraftTweaker that allows loot tables to be tweaked with ZenScript files. The vanilla loot table system is limited to overriding existing loot tables or creating entirely new ones.
LootTweaker allows you to make small or large tweaks to loot tables, without having to override the table completely.

Documentation
Documentation can be found here.

FAQ

Will there be a 1.13/1.14/1.15/... version?
LootTweaker will be remaining on 1.12.x. The CraftTweaker team is making excellent progress towards support for the post-1.13 loot system. Therefore, rather than spending a lot of energy on understanding the post-1.13 loot system and creating a useful ZenScript interface to it, I have chosen to continue to improve LootTweaker for 1.12.x.

Can LootTweaker edit mob/structure/whatever loot?
LootTweaker edits loot tables. If a vanilla mechanic or mod mechanic uses loot tables, LootTweaker can edit those tables. To find out if a mod uses loot tables, check its documentation or ask the author.

Can LootTweaker create new loot tables?
Yes, this feature was implemented in version 0.2.0. See the documentation for the class loottweaker.LootTweaker for details.

Can LootTweaker automatically find and remove all of an item from loot tables?
No, though the feature is being worked on. There is no estimate for when it will be ready.

Can LootTweaker be installed only on the server?
Server-only installation is possible with LootTweaker 0.1.7 or later. However CraftTweaker requires that it is installed on both server and client. Clients without LootTweaker installed will still be able to use LootTweaker commands, but the feedback messages will be untranslated. Note that loot table dumps occur serverside. Client-only installation of LootTweaker is not supported.

What happened to LootTweaker 0.0.9?
0.0.9 was extremely broken, crashing any dedicated server that used any of its functionality due to an oversight on my part while trying to improve the warning and error messages. Initially I archived it, making it unavailable for direct downloads, but still available to packs that were using it. However, I've received many requests from users for 0.0.9 because servers they are playing on use it, so I've deleted it. It's really broken, update to 0.0.10.

Important Notes:
Modifications made by LootTweaker are not detected by Just Enough Resources prior to 1.12. The cause of this issue is unknown, neither I nor the JER author could figure it out. Only LootTweaker for 1.12 receives new features. Due to large differences in the codebase caused by the port to 1.12, I can no longer develop LootTweaker for all versions simultaneously. LootTweaker for 1.10 and 1.11 will continue to receive bugfixes however. A Forge version of at least 14.23.5.2779 is required

Want more loot conditions/functions?
Take a look at More Loot Stuff. If the condition/function isn't already in More Loot Stuff, suggest it and I'll see what I can do.

Need help or have a question?
I have a discord server with a dedicated channel for LootTweaker. You can of course also contact me here, but I'd advise going to the discord server as you can get help there when I'm otherwise occupied.

Have a suggestion?
Create an issue hereor just comment on this project.