Sulfur

or Sulfur (Natura). Sulfur is a non-metal added by XyCraft, Railcraft, The Vegan Option and GregTech. Natura and Thermal Expansion also adds it's own sulfur variant, but this version is not interchangable with the others, whereas the sulfur from the first four mods can be used in each others recipes.

Sulfur Ore (Added by Railcraft) only spawns in Extreme hills biomes around lavalakes, 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: