Slab Machines aims to conserve space by making slabs out of the most common machines in Minecraft. It cuts the chest down to half the size while maintaining the same amount of storage. And they are all stackable so you can more than double the storage in the same area as opposed to traditional double chests.
You can also increase the amount of smelting done at the same time with slab furnaces. These do cut the amount you can smelt to 32 maximum items but you can do more varieties in a smaller space by stacking as well.
This mod also introduces slab TNT blocks that do about half the damage as regular TNT.
And you can cut the amount of space a crafting table uses as well. Plus all of these slabs are stackable with each other so you can take up a significantly smaller footprint in your structures by combining space with these slabs!