NPC mods are in high demand for Minecraft because many players don't want to play on a multi player server (Minecraft politics can be a headache) but at the same time they don't want to feel as if they're playing in the aftermath of a great nuclear winter in which only cattle, sheep, pigs and monsters have survived.
If you want to add some NPC's to your game, here are five great mods that I have play tested and verified awesome.
Minecraft Builders is at the top of the list because I think it does the most in terms of changing the Minecraft experience and really giving you the feeling that there are autonomous NPCs out there in the world with their own business to attend to.
You can read more about this mod by clicking the link above, but suffice to say, Minecraft Builders adds new mods to the game that autonomously build structures, including houses, trading shops and town walls. This mod is compatible with Humans +
Mine Colony is a Minecraft mod that adds four NPCs. There is a farmer who will grow wheat, a miner who will mine, a lumberjack for all your wood chopping needs and a delivery man who goes between the previously mentioned mobs, picks up the fruits of their labors and delivers them back to a warehouse.
This is an awesome mod if you want to automate things like wood collection and wheat farming, or even if you simply enjoy the pleasure of watching other people work. In this mod you can spawn the above NPCs by creating special chests. When you place their chest they pop out of it like a genie and get to work.
There are also two scepters which can be crafted. Wave a golden scepter at a NPC chest and a hut will magically appear. Wave an iron scepter and a fence will pop up around their work space (usually 20 blocks in every direction.)
Fairies might not seem like a 'cool' mod because at first glance all it does is add a bunch of giggling crooning fairies to your world. It is a most excellent mod however, because you are able to tame Fairy NPC's and make them ride your head. With a fairy on your head, you can jump two blocks instead of one and also glide to safety.
Humans + adds a bunch of human NPC's to the game. It's pretty hilarious as not all of them get along so it's not uncommon to see battles taking place in front of you. There are also pirates who will steal your boat and row out into the middle of large lakes, so that's nice.
Forever alone Minecrafters can finally have the girlfriend of their dreams with the girlfriend mod. This mod was designed by Kodaichi, the same creator who graced us with Minecraft fairies. Charlotte will follow you around on your adventures and help you when she can. If you die however, she will not recognize you when you respawn and will simply run around crying. Tragic really.