Gakumini is a mod that adds small additions to Minecraft that build up. Simple as that.

String recipe (grass or tall grass in any crafting slot gives 1 or 2 string, respectively)
Saltpeter (a new resource that appears at -10 to -50 in the Overworld, or in the Nether. Fairly abundant.)
Gunpowder recipe (requires charcoal, glowstone dust and saltpeter)
Shields have a short cooldown after getting hit by anything (configurable with a gamerule).
- Attacks that normally disable shields are classified as "strong" attacks, and inflict a longer cooldown (also configurable with a gamerule).
Wither skeletons drop Witherbone, a new item which can also be crafted with bone, saltpeter, coal and charcoal.
- Witherbone can be used to craft Wither Skeleton skulls, with 8 around a golden apple. Less tedious.
- Witherbone can be used to craft Blaze Rods.
Magma cream can be crafted by blasting magma blocks.

1.2.0

Coins. They can be signed, so you can limit their quantity, with different emblem options. (This is taking more time than expected, so please have some patience with this update)
Totem Sickness, a new effect that can't be removed with a milk bucket. It applies when a player uses the Totem of Undying, and prevents it from being used until the effect wears off.