Sulfur

Sulfur is a non-metal and an element added by various mods. Natura and Thermal Expansion 3 also adds their own sulfur variants, but their versions are not interchangeable with the others.

Vanilla
on the Minecraft Wiki

Gaspunk
Sulfur can be randomly found in nether fortress' chests and is used to brew Mustard Gas grenades. It is added to the Ore Dictionary with the tag.

IndustrialCraft 2
Sulfur is currently only used to craft the.

Mekanism
Sulfur can be extracted out of Coal with the PRC. For this process the PRC requires Water and Oxygen.

NuclearCraft
Molten Sulfur is created by passing Sulfur through a.

Railcraft
Sulfur Ore only spawns in Extreme Hills biome around lava lakes, but it is extremely common there.

The Vegan Option
Sulfur has a 2% chance to drop from Netherrack when broken. It can used to craft Gunpowder, with the following recipe:

Thermal Expansion
Sulfur is a byproduct of pulverizing Nether materials in the. It can be used to create Gunpowder or can be further pulverized to create Sulfur from Factorization.