Liquid

Mods in Minecraft have introduced many new liquids beyond vanilla Minecraft's water, milk, and lava.

BuildCraft

 * Oil
 * Fuel

Forestry

 * Liquid Honey
 * Seed Oil
 * Biomass
 * Ethanol (Formerly Biofuel)
 * Short Mead
 * Apple Juice

Railcraft

 * Creosote Oil
 * Steam - Not a liquid in a complete sense since it can't be picked up with buckets, but it can be stored in tanks and pumped in Waterproof Pipes or Liquiducts.

Thermal Expansion

 * Molten Redstone
 * Molten Ender
 * Energized Glowstone

Unsorted

 * Acid
 * Poison
 * Liquid Nitrogen
 * Crushed Ice
 * Coolant