Tekkit is a mod compilation. It has a number of big mods, and a bunch of smaller ones.
For big we have...
Industrialcraft: Electrical power, machines that do everything from grind ore into dust, to generate matter out of thin air (and HUGE amounts of power), alongside some nifty pieces of gear.
Buildcraft: Pipes and engines and automatic mines. Buildcraft is mostly about getting things from one place to another, be it from the ground to your chests, or from your chests into the furnace, it lets you automate a lot of things.
Equivalent Exchange: Transmutation of matter. Everything has a value, and if you have two things of equal value (say, a stack of dirt and obsidian, or four gold and a diamond) and you can trade one for the other. Latter on you can get stuff for nothing, and make extremely expensive, but utterly amazing items.
Then you got the small stuff. Treecapitator, which makes the whole tree fall if you hack at it with an axe, mo creatures, which adds things like rabbits and lions, millinaire, which adds villages of people, and a bunch more things.
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