I recently became re-addicted to Minecraft after taking a break for a while. I did start playing a regular world on the snapshot a couple weeks ago but then I decided to finally start delving into mods. I made a new world... this time with IndustrialCraft, BuildCraft, RailCraft, RedPower (with a add-ons for those like Advanced Machines, Logistics Pipes, etc), Thaumcraft, Equivalent Exchange, Portal Gun, Forestry, Iron Chests, and a couple of smaller mods like NEI. I was majorly influenced by
DireWolf20's Let's Play Season 4 series on Youtube, I'm not using everything he has but I do have the majority of it.
Holy shit, the game is instantly more crazy than I ever imagined. I'm so impressed with what these mods have put into the game and how well everything just kind of works together. Using Buildcraft to build a quarry, that uses Railcraft to dump items into a chest cart, that travels through a Portal mod portal, to my base where it's unloaded by logistics pipes and then passed through all the IndustrialCraft and Equivalent Exchanges machines only to finally be sorted into Iron Chests so I can later research it with ThaumCraft to build myself magic carpets, lightning swords, and necromancy wands is awesomely satisfying. Won't even get into the fun with RedPower and Portal Gun antics.
It just gives you so much to do and so much to aspire to and spend your resources on. I've barely even touched Forestry outside of bee keeping and am only like 15% through what Thaumcraft has to offer. God forbid I go and install another mod like MystCraft or Doggy Talents.