Placebo is an MCForge exclusive library mod that is used for a large selection of mods such as:

Apotheosis
Attained Drops 2
Click Machine
ComPatchedStorage
FastFurnace
FastSuite
FastWorkbench
Gateways to Eternity
Growable Storage Cells
HexxitGear
HitWithAxe
Hostile Neural Networks
Ignition: Enderbags
Ignition: Foundry
Ignition: Hydrogel
Magical Map
Magical Map OC Edition
Pack Menu
Plants
Singularities
Toast Control
Wither Skeleton Tweaks
And more.

As the name suggests, the mod does nothing when alone but is required for many of the top mods to run.

Many of the mods for Placebo are made by Shadows_of_Fire (the person who created Placebo), but third parties create many other mods, and you are free to use Placebo for your mods too. Some of the features provided by Placebo include:

Chest Utilities
Task Queue
Random Range
Registries
Attribute Utilities
Enchantment Utilities
Maths
JSON tools
Config tools
Event Management
Loot
Recipes
Networking
Commands
Colours
And much more

Placebo is one of the largest libraries for Minecraft modding; these are just a few of the many features it provides to mod makers, though there are some things to keep in mind about using it. First of all is that while others are allowed to use it for their mods, it, in many ways, was designed for the mods of the creator so that those things will have priority; another thing is the documentation is pretty sparse, and you will need to learn how to use many of the tools and utilities yourself. Even though the Javadoc is very little, another big thing is that Placebo is officially only supported on Minecraft. Forge mods made in it will be limited to their mods running on Minecraft Forge. An unofficial fork by BJTMastermind exists, but it is unlikely to be officially supported.

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Placebo Mod