Version 2.2.0 - Recommended
Some additional utility pipes for BuildCraft.
Force fields, hostile mob management, etc.
Upgraded versions of the basic IC2 solar panel.
Website: http://ae-mod.info
Advanced item storage, processing automation and crafting.
Core mod used by Chisel.
Need more inventory space? Backpacks are your answer.
A metric ton of custom weapons. If you really needed a cannon, this is your mod.
Website: http://www.big-reactors.com/
Multi-block nuclear reactor structures.
The core library used by the rest of bspkrs' mods.
Website: http://www.mod-buildcraft.com/
Item transport, automated mining, power production. Something for everyone.
Website: http://www.minecraftforum.net/topic/1790919-1516forge-carpenters-blocks-v202-slopes-stairs-and-more/
Use almost any block as the texture for fences, doors, slopes, stairs, etc.
Keep chunks loaded when a player is not present in the area.
Additions of many new ways to "cut" blocks for appearance.
Core library for all Chicken Bones' mods.
Lib for ChickenBones mods 1.8+
Power production and storage, automated processing, energy/item/liquid transport. A cornerstone mod.
Website: http://www.computercraft.info/
Write programs in LUA on computers and mobile computers called turtles which can interact with the world.
Website: http://www.minecraftforum.net/topic/2076108-164smpcrafting-suite-now-with-plans-v0213forge/
Fancy crafting tables with inventories, tool slots, custom textures and other goodies.
Website: http://immibis.com/mcmoddl/
Keep chunks loaded when no players are present in the area.
Link chests and bags through the ether and across dimensions.
Convert stuff into other stuff.
Extra Bees and Trees for Forestry.
Adds liquid storage to Applied Energistics ME Networks.
Add many new biomes, trees, plants, etc. to the vanilla worldgen.
Custom material processing, storage and automation.
Adds forestry management to MC with trees, bees, resource processing and much more.
Fixes Java 8 incompatibility with earlier versions of Forge.
A library providing the ability to cut up blocks into smaller pieces, and an API which mods can use to have two blocks in the same block space.
Go to the moon and mars in a rocket! Yeah, it's as good as it sounds.
Core library used by other mods by heldcore.
Add control blocks for nuclear reactors, information panels and many other utility features for IC2.
Fixes the NEI recipe display for broken IC2 recipes.
Website: http://immibis.com/mcmoddl/
Core library for the rest of Immibis' mods.
Website: http://immibis.com/mcmoddl/
Another microblock mod. Works "in the same block space" for a few blocks that Forge Microblocks doesn't cover.
Website: http://immibis.com/mcmoddl/
Advanced IC2 machines that don't suck.
Machines and mechanisms for automated processing, energy production and a slew of other features. A cornerstone mod.
Automatically replace items/blocks/tools into your hotbar when they get used up.
Bigger, better, more expensive chests. Giggity.
Config files for JacksonCraft modpack.
Fixes Forge on Java 8 for 1.6.4 and 1.7.2
Advanced item and crafting management for BuildCraft.
Machines for processing, energy production.
Automated factories for plants, animals and everything in between.
Website: http://www.minecraftforge.net/forum/
Minecraft Forge is a Minecraft API which allows almost maximum compatibility between other Forge mods.
Core library support for Waila & Opis
Website: http://calclavia.com/mffs/
Keep those mobs, and maybe players, at bay!
Advanced armor with a plethora of modules to make your live less frustrating.
Website: http://binarymage.com/
Adds the ability to create "ages" which are worlds with custom properties. Large, fun, complex mod.
Adds many new trees and flowers to the vanilla world generation.
Website: http://bdew.net/neiaddons
Adds features to NEI.
Add NEI information for Mystcraft.
Another set of additions to NEI.
Adds an interface on top of your inventory that lets you browse all blocks and items which Forge/MC knows about.
Library used by MMMPS
A collection of fancy pressure plates.
Sensor additions for ComputerCraft.
Website: http://www.openblocks.info/
Core library used by OpenPeripheral.
Website: http://www.openblocks.info/
Adds additional mod APIs to OpenPeripheral.
Website: http://www.openblocks.info/
Allows ComputerCraft to treat many blocks from different mods as peripherals.
Website: http://www.minecraftforum.net/topic/2104497-164server-admin-opis-112-alpha-now-with-entity-support/
Server profiling mod. Track down that lag.
Adds a ton of video settings and options with the hope of improving performance.
Convert between many different energy systems.
Core library for other PowerCrystals mods.
A Red Power replacement with extra goodies.
Website: http://www.railcraft.info/releases
Automate all the things.. with railroads!
Redstone Flux infused tools.
Dependency mod for MFFS.
Used by MMMPS.
Website: http://stevescarts2.wikispaces.com/
A different take on automation with carts. Mining? Check. Farming? Check. Awesome? Double-check.
Power production, material processing, transport and much, much more.
Website: http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods/2261817-tinkers-mechworks
Movable bridges, gates, etc.
Website: http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods/2218638-tinkers-construct
Adds an entire forging system with multi-block smelteries and a plethora of custom tools and weapons.
Much, much more TNT. Make houses, change the landscape, spawn mobs and make REALLY big holes.
A trade booth. Buy and sell.
A simple way to move item and liquids between inventories.
Chop down trees with a single blow! Well, a lot of blows. Better have a good axe!
Provides some utility blocks like an incinerator.
Website: http://universalelectricity.com/
Core mod for other Calclavia mods.
Website: http://www.minecraftforum.net/topic/1116564-164-162-mamiyaotarus-mods-voxelmap-chatbubbles-etc/
In-game minimap.
Adds a HUD style tooltip for information about "What Am I Looking At".
Wireless Redstone. Pretty straight forward.