Sulfur

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

IndustrialCraft 2
Sulfur in IndustrialCraft 2 is currently only used to craft the.

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

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

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