Mods

Mods (short for modifications) are changes, additions, or deletions to Minecraft that alter the original gameplay. Mods that add new content to the original game are often called partial conversions, while mods that create an entirely new game are called total conversions, and mods that fix bugs are called unofficial patches.

The Minecraft mod industry has been growing, and better and better mods are being seen. However the mod that tops them all is the iMod. The iMod adds much new content to Minecraft, including multiple new mobs, peaceful and hostile. If you care to check it out, you can find the forum here: http://www.minecraftforum.net/topic/855313-100-imod-v3-0-try-2/

Mods for Minecraft can introduce an entirely new experience and increase the creativity in many different ways, and the Mojang team plans to add an official modding API that allows modders to more easily add content to the game.

Most mods in Minecraft add content to the game to alter gameplay, change the creative feel, or give a player more options in how they interact with the Minecraft world. Some may be bigger expansions, such as the Aether Mod, that add a significant amount of new content to the game. Other mods take away content to create a simpler or more specific gameplay experience. Some mods, such as the Portal Mod, attempt to bring over another game's content, experience, and feel. And still other mods do more technical things such as adding or expanding more settings/options or optimizing the speed, graphics, or gameplay of the game. Server mods or plugins mainly give server admins more options and ease of use, and most mods for single-player have a server version that allows or optimizes the mod in multiplayer.

Ideally, the creator of a Minecraft Mod updates the mod whenever the game updates - bringing more content, bug fixes, or optimizations. Otherwise, the mod may not work with a new version. Many in the Minecraft community appreciate the additional experience and ease of use that come with mods, while others play "vanilla", the original game, with only the original content, and might view mod usage as cheating. While Minecraft mods are generally safe to install, one should exercise caution with mods to prevent crashes, deletion of game or save data, system instability, or potential malware infections from a bad link or the mod itself. There are many mods or plugins available to complement the original Minecraft game or give server admins more options and control over their servers.

Harmful Mods and Mod Safety
Due to the fact that mods can execute almost any arbitrary code, it is possible a mod could deliver a virus, steal your account information, or cause other harm to your computer if the creator had that intention. There aren't many cases of this happening but it's worthwhile to look for community feedback about a mod before installing it.

If you choose to trust a mod, make sure to download it only from its official download location. The nice guy putting up a fast mirror of YourFavoriteMod may have added a little extra something....

Mods can also inadvertently cause issues with your minecraft installation if they aren't properly made and can potentially cause issues to save files. Again, this isn't very likely with established mods, but it would be wise to back up your .minecraft folder or saves folder before using a new mod.

Multiple mods may conflict with each other; certainly if two mods need to alter the same files, then they are unlikely to be compatible and will not work.

Mods generally need to be run with the exact Minecraft version they are developed for; mismatched Minecraft/mod versions are unlikely to work. Even when updating both Minecraft and your mod(s) at the same time, there still may be issues with saved worlds, so it is strongly recommended to backup your saves before any update.

Outdated Mods
A full list of outdated mods can be found here.

Client Mods
Client Mods are modifications to your Minecraft program (Alpha or Beta). They are not custom clients, and require modification of minecraft.jar. Like high resolution textures, they only work with the launchers, and won't work if you play Beta through a browser.

The most common form of client mods are aesthetic and/or functional. However, some client mods add content to the game, and others fix issues with the game. While some do work on SMP, mods that add content, and thus  could be used for griefing, do not work, as the server will ignore  custom content.

Mod List
Note to people editing this page to post their own mods:

First, please put your mod in the correct section. If it adds new mobs, items, or blocks (or dimensions), put it in New Content. If it changes features of an existing item or adds settings, like adding a new difficulty level or making mobs drop more experience, put it in Aesthetic and Functional Mods. If it fixes a bug in "vanilla" Minecraft, put it in Fixes. Second, please keep the mods in alphabetical order. It's much harder to find older, established mods when they are buried beneath the mods that have been thrown on top of them.

Thank you for following these simple rules.

APIs and Loaders

 * For a list of Outdated APIs/Loaders, see here.

Fixes

 * For a list of Outdated Fixes, see here.

Aesthetic and Functional Mods

 * For a list of Outdated Aesthetic and Functional mods, see here.

New Content
Any mods that add new blocks, items, or mobs will have new Data Values, which can be found on the Mods/Data values page.

For a list of Outdated New Content mods, see here.

Introduction
You have to delete the META-INF folder inside the file "minecraft.jar" and replace the files "en_US.lang" and "stats_US.lang" inside the folder "lang" in order to use a language pack.

You can find a complete installation guide for different operating systems on the Minecraft Forums.

As of snapshot 11w49a, language packs are obselete, due to the official implementation of other languages.

Server Mods
Server Mods are modifications to the official Minecraft server software. They generally fall into two broad categories: management and gameplay.

Management mods are commonly designed to make administration of servers easier by implementing tiered privileges for commands (such as kicking, banning etc.). They are frequently implemented as "wrappers" which do not actually modify the main server .jar file, instead monitoring its output and sending commands to it.

Gameplay mods are modifications to actual game mechanics, such as restricting the placement of blocks commonly abused by Griefers (e.g. lava, TNT), fixing bugs in the server (e.g. making furnaces function before the update of 10th of September 2010) or adding new gameplay features.

Mod List
This is a list of wrappers and modifications. For server mods for out of date versions, see here.

Server Operating Systems
Server OS'  are often Linux distributions specifically built to host a Minecraft server.

They are built to ease the burden for Minecraft server administrators by providing tools to manage deployment & maintenance.