SFX Toolbox is a modpack centered on deepening the world of Minecraft in several aspects, in order to make the game more enjoyable. There are still a lot of improvements to be made: see the issues tab to see what is currently being worked on, and you can even suggest your own changes there as well.

Notable Mods

Lithium and others to improve game performance.
Immersive Portals, Doors of Infinity, and Plato's Transporters allow new kinds of constructions to be made.
Simplex Terrain, Better Nether, Terrestria and more provide a rich world to explore.
Tech Reborn, TIS-3D, Art of Alchemy and others allow complex magic and tech systems to be created.
Good Ol' Regen, Spice of Fabric, Crimson Moon, and more revamp difficulty.
Blockus, Extra Parts, and others to give more blocks and shapes for building.
Croptopia for cooking, and other hobby mods.

Credits

All credit for the mods goes to the mod authors, see the 'relations' tab for links to their mods.

A few resource packs have been included as well:

Dark GUIs by Marioluigi7896
Motschen's Better Leaves by Motschen
Round Trees by Motschen
Subtle Glint by Vespilan