A list of mods which are useful for modpack makers

pack making mods

Mine/Craft Tweaker - Create/Change crafting table or furnace recipes.
Content Tweaker - Create new blocks, items, liquids and much more.
Mod Tweaker - Adds mod compatibility to Craft Tweaker.
Game Stages (And addons) - Prevent players from accessing dimensions/items/mobs/materials/waila information before they reach a certain point.
Gendustry - Create custom bees, combs, bee mutations and squeezer/carpenter recipes.
Block Drop Tweaker - Change what certain blocks drop.
Custom Stuff - Create custom items, blocks, recipes, world gen and oredicts.
Custom Things - Create custom items/blocks.
Modular Machinery - Create custom machines.
Artisan Worktables - Adds many variations of the crafting table and many more crafting tools.
Custom Main Menu - Customize your main menu.
Resource Loader - Load resources into minecraft.
Extended Crafting - Introduces new crafting methods.
Recurrent Complex - Generates custom structures during world generation.
Biome Tweaker - Tweak biomes or create new ones.
Biome Tweaker Core - Allows Biome Tweaker to work better/do more.
YUMakeNoGoodMAP - Generates a void world with a floating island (Mainly for skyblock packs).
ItsBecauseUHasNoGoodSpawn - [YUMakeNoGoodMAP Addon] Used to generate islands for more players (Mainly for servers).
Advanced Rocketry - Adds a lot of astronomical stuff and it can be used to create custom solar systems and planets.
Dimensional Control - Tweak dimensions.
Material Changer - Makes it possible to tweak tools/armor.
CoFH World - Can be used to change the way ores generate.
Custom Starter Gear - Gives item to players when they first join a world. 
MiscTweaks_ - A variety of options.
Twix - Can be used to create custom advancements.
Simply Gems - Adds a bunch of gems (and ores) which can be used for making custom recipes.
Enchiridion - Can be used to create custom books.
Hardcore Questing Mode - Adds a questbook, the ability to create quests.
Better Questing - Another option for creating quests.
Artisan Tabs - Control over creative tabs.
Custom Backgrounds - Custom menu backgrounds.
Guidebook - A way to explain to people how your pack works.
Guide API - An alternative o enchiridion.
Pillar - Generate structures in the world with the use of structure blocks.
Musica - Allows pack makers to create custom music disks.
LaunchGUI - Displays a message every time the modpack is launched.
Placemod - Generates schematics throughout the world.
Infini-TiC - Register materials for Tinker's Construct.
Ambience - Can be used to load music into minecraft.
StructPro Mod - Generates schematics throughout the world.
Open Terrain Generator - A very flexible world generator.
Pickle Tweaks - Various options.
Improved Mobs - Slightly tweak the way mobs act/work.
Tweakers Construct - Tweak materials for Tinker's Construct.
Block Tweaker - Tweak properties of any block.
It's the little things - Various details.
Crafting Harmonics - Recipe configuration.
Progression - Allows you to control how a player can interact with the world, or the things they can craft, at the basic level.
TooMuchRain - Configure rain.
WeatherConfig - Configure weather patterns.
MobSpawnerControl - Configure mob spawners.
InControl! - Tweak where, when and under which conditions which mobs are allowed to spawn.
Custom Starter Gear - Give your players a starting inventory.
Ore Controller - Change or add new stuff to ore generation.
Mob Controller - Limit mob spawning.
CaveControl - Change how often caves generate.
Void Island Control - Useful for skyblock packs.
Climate Control/Geograpgicraft - Tweak humidity and temperature of every biome.
Evil Ocean - A world generator where the entire world is mostly water and players can drawn instantly.
Finite Water Control - Disables water generators
LonelyBiome - Allows you to make worlds covered by a single biome.
World Generation Loader - Customizable world generators.
Biome Height Tweaker -Adjusts height variance.
Biomes Tweaker - A simple way to tweak some biomes.
Sky Island World Generator - Simple skyblock generator.
Tweak the Properties of any Block - (Read the name)
Ore Tweaker - Configure ore generation.
Advanced mortars - Adds 4 mortars and a new crafting system. There are no recipes by default but it has CraftTweaker support.
Initial Inventory - Gives items to new players. Uses zenScript.
Just Enough Dimensions - Some control over dimensions. Allows you to change the starting dimension.
Additional Loot Tables - Tweaks loot tables.
ToolTweaks - Prevents players from using certain items/tools.
Custom Tweaks - Small and configurable tweaks.
Guidebook - Similar to enchiridion.
Fabricate - An alternative for CraftTweaker
Tinkers Extras - A good amount of configurations for TiC parts.
Artisan's Tabs - Can be used to create your own creative tabs and populate them with items.
EFab - A crafting system with many opportunities for gating.
Bigger Crafting Tables - Adds 5x5 and 7x7 crafting tables with craft tweaker support.
Botania Tweaks - Allows you to tweak various botania stuff.
Pedestal Crafting - Infusion Altar(TC)-like crafting system.
Scavenge - Customizable block interactions.
Rocky Core - Anvil recipes and villager trades.
Packmode - Have different scripts for each mode of your pack (easy, normal, expert, etc.).
Tree Tweaker - It adds customizable trees (+saplings). 
FTB Quests - Questing mod.
Triumph - Achievement-like quests.
Thaumcraft Research Loader - Custom thaumcraft research.
Sift - ExNihilo-like sieving.
Ced's Unleashed Control - JSON customizations.
Research Table - Game Stages addon which allows pack maker to add custom research system based on stages.
Alfinivia - CraftTweaker addon for milking, liquid interactions, break speed, damage modification, misty world methods, and IE methods.
Simple Grinder - (No explanation needed).
Patchouli - One of the greatest guidebook mods so far.
Mana Tweaks - Allows CraftTweaker scripts to request mana from, or provide mana to, items in the player inventory.
Quadrum - Easy way of creating custom blocks/items but less advanced than CoT.
EFab CT Bindings - CraftTweaker support for EFab.
CraftTweaker Utils - CraftTweaker addon.
Delivery - Adds a store for modpack makers to customize.
Galacticraft Tweaker - CraftTweaker support for some Galacticraft machines.
YouAreAnExpertHarry - Easy way of making recipes for an expert pack ( https://github.com/quat1024/YoureAnExpertHarry ).
Watermark - Displays configurable text on the top of the screen during gameplay.
ExplosionCrafting - Stuff + Explosions = Other stuff.
InWorldCrafting - Fluid + Item recipes.
Sound Reloader - Reloads sound files.
Resource Reloader - (Re)Loads a specific texture instead of all textures.
IC2 Tweaker - CT methods for IC2
Pewter - Allows for custom TiC materials. (Also adds materials for some magic mods by default)
Sips - Gives you the ability to make every fluid drinkable.
Advanced Rocketry Tweaker - A way of working with AR recipes with Craft Tweaker.
ZenTweaker - 15+ utilities which might be useful for modpacks.
Enchantments Control - Almost complete control over both vanilla and modded enchantments.
SpawnTableTweaker - CraftTweaker methods for changing spawn tables.
Heat&Climate Tweaker - Added integrations for most machine in Heat&Climate.
Custom Capacitors - This mod allows you to define custom power storage blocks using json files.
Tiered Depths - This is a crafttweaker addon that allows the packmaker to restrict mining levels based on the y level.
RitteClicker Mod - This mod adds the right click dirt or grass functionality first introduced by Vazkii in the Botania Garden of Glass mod.
Alembic - Adds a command which gives specific thaumcraft research to players. Intended for use on quest rewards.
Groves - Adds ZS methods for customizable mob spawners (not in the vanilla sense).
MoreTweaker - CraftTweaker support for even more mods.
Extra Utilities 2 - Allows for the creation of custom, single-block machines (but also adds stuff).
Requious Frakto - Allows for the creation of custom, single-block machines.
Mystical Creations - Using it you can create Custom Seeds for your modpack. It generates for you the seed, crop, essence and mob chunks. It doesn't add any recipes.
Edit Mob Drops - Title.
Mob Drops Your Way - Allows you to add anything in game as a drop to any mob. You are able to add drops for player only kills or drops from any random kill.
Loot Tweaker - CraftTweaker addon which allows loot tables to be tweaked, using ZenScript.
HungerTweaker - Can be used to modify food, and hunger.
Zen Summoning - Adds an "altar" block which summons entities when a specific set of items is given to it. CraftTweaker must be used to add "recipes".
Zen Toolforge - The mod includes two significant functions and three minor ones. The two significant ones let CraftTweaker build a tool from a tool definition and a list of materials, and deconstruct tools into their component parts.
Reskillable - Can be used to lock stuff behind skills.
CompatSkills - Mod support for Reskillable.
LootTableTweaker - Allows CraftTweaker to interact with the loot table system.
Thicc Entities - Provides a config which lets you change the scale, attack strength, speed, jump height, and such of nearly any living entity in the game.
Loot++ / Additions Mod -  Lets you add your own content to the game (items, blocks, recipes, discs, and more)
AuraControl - Thaumcraft aura restrictions.
Erosion 2 - Certain blocks will erode when there is water above them (Configurable).
Restriction - This mod restricts placement and interaction (right click) of blocks/machines.
ArmoreableMobs - Give any item to any entity when it spawn using CraftTweaker. 
Own Crops - Adds custom crops.
Default Dimension - Change the starting dimension.
Controlled Burn - Customize fire blocks.
Topography - Custom terrain generation/dimensions.
Natural Absorption - Adds a highly configurable absorption-like mechanic, along with an enchantment for increasing it.