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.

Sulfur Ore (Added by Railcraft) only spawns in Extreme Hills biomes around lava lakes, but it is extremely common there.

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: