Mods introducing new content into the game.
Large total conversion mods.
Small changes concerning balance, gameplay, or graphics.
Providing the player with new tools or adjusting the game interface, without fundamentally changing gameplay.
Scenarios, maps, and puzzles.
Collections of mods with tweaks to make them work together.
Translations for other mods.
Lua libraries for use by other mods and submods that are parts of a larger mod.
Transportation of the player, be it vehicles or teleporters.
Augmented or new ways of transporting materials - belts, inserters, pipes!
Trains are great, but what if they could do even more?
New ways to deal with enemies, be it attack or defense.
Armors or armor equipment.
Changes to enemies or entirely new enemies to deal with.
Map generation and terrain modification.
New Ores and resources as well as machines.
Things related to oil and other fluids.
Related to roboports and logistic robots.
Entities which interact with the circuit network.
Furnaces, assembling machines, production chains.
Changes to power production and distribution.
More than just chests.
Change blueprint behavior.
Play it your way.
Alternative Factorio, slightly changed rules, primarly to be more scientifically correct. Mass storage building, substations have a large supply area, new chemical ammonium nitrate, more use for wood, charcoal recipe, sulphur is gained from oil and coal instead of gas. Additionally makes building up and planning in the earlier stages easier. Don't use on existing savefiles, mixing with other technology tree changing mods will probably make the game unplayable.
Small changes concerning balance, gameplay, or graphics.