Farms: mobs, herbs and Life Fruit

Mob farms

Stack a Water Candle (crafted at Crystal Ball) and a Battle Potion (found in chests Shadow Chest (Underworld)) for more enemies: Increases enemy spawn rate.

Build away from your town, or place a Shadow Candle (crafted at Work Bench): Nullifies the peaceful benefits of towns. The Rod of Discord (dropped by Chaos Elemental) gets you in and out fast.

Statue farms

Wire a statue such as the Slime Statue (placed in new worlds) to a 1 Second Timer (crafted at Anvil) with Wire (sold by Mechanic); each signal spawns one creature. Add Dart Trap (placed in new worlds) to kill them.

Herb farms

Plant herbs such as Blinkroot (from Blinkroot) and Glowing Mushroom (from Mushroom Plants, Giant Mushroom and 1 more) in Clay Pot (crafted at Furnace), and harvest with the Staff of Regrowth (found in chests Ivy Chest): Creates grass on dirt. Increases alchemy plant collection when used to gather.

Life Fruit farm

The Life Fruit (from Life Fruit) (Permanently increases maximum life by 5) grows on underground Jungle grass in Hardmode after a mechanical boss. Lay a flat strip of Mud Block (from Jungle Grass Seeds, Mushroom Grass Seeds and 2 more), plant Jungle Grass Seeds (from Jungle Plants Echo, Jungle Plants and 1 more) on it (they go only on mud), and let it grow.