IndustrialCraft 2 [A classic tech/industrialisation mod! Auto-Crafting and more!]

BuildCraft [A simple mod to get into, but just as cool as many other tech mods. Build huge quarries and let them do the hard work for you!]

Steve's Carts 2 [Improve your rails beyond belief and much more!]

GalactiCraft [Travel to space!]

PSI [Program your own spells!]

Tinker's Construct [Make your own custom tools!]

EnderIO [Compact conduits, machines, powerful tools and armor and more!]

Thermal Expansion [New upgrades, tech and electronic mechanisms!]

Iron Chests [Larger chests]

Immersive Railroading [Awesome new trains for transportation of all kinds!]

XL Food [Over 150 new foods and items!]

Applied Energistics 2 [Make storage systems like never before!]

Twilight Forest [Explore a whole new dimension with new enemies, biomes and bosses!]

Botania [Natural magic!]

Quark [A bunch of awesome little tweaks and additions!]

Blood Magic [Make powerful spells and items with blood!]

Immersive Engineering [Community favorite that adds a bunch of tech-based features and items, with a focus on realism!]

Electroblob's Wizardry [Become a Master Wizard!]

And also some of my personal favorites!

Optifine [Makes the game run better, no cons!]

Mekanism [Tons of cool machinery!]

Minecolonies [Make your own colony and build up an empire!]

JEI and Hwyla [Find items and recipes much faster!]

Aether Legacy [An updated version of the original Aether mod which adds a whole new sky dimension to explore and conquer!]

...And many more!