Basic Fluid Hopper

The fluid hopper is similar to a hopper, but it moves fluids instead of items. A minecart version is also included.

The fluid hopper interacts with a few other blocks to make it far more useful.

Fluid Sources

Drain water, lava, or other fluids directly from the world. This will only work for fluid sources directly above the hopper, so it would be most useful underneath infinite sources.

Cauldron

A hopper can pull lava or water from a cauldron, or push water or lava into a cauldron.

Combine with a fluid tank (such as "Large Fluid Tanks") to collect a lot of lava from a dripstone farm.
Pull from an infinite water source to keep a perpetually full water cauldron useful for filling water bottles.

Furnace (or Blast Furnace, Smoker, or many furnace type blocks from other mods)

A fluid hopper can directly insert lava (or any other fluid with the c/fluid/fuel tag) into a furnace on demand. Connect a lava dripstone lava farm directly to a furnace and never worry about moving fuel around again!

Beehive

Place a fluid hopper under a beehive, and honey will automatically be pulled out when it is made.

Dispenser / Dropper

Face a fluid hopper into a dispenser or dropper, and the dispenser/dropper will fill empty bottles (with honey or water) or buckets (with any supported fluid) when it dispenses them. Droppers will also fill empty bottles or buckets when inserting items into other containers.