Mods

Mod Safety
You must never try to install a mod into the game or server unless it has been updated to exactly match the version of the client you are running (see the Latest Compatible Release column below). Even a slight variation could be enough to cause problems, including crashing or corrupting your saves/world.(NOTE:If you name your world the same as it used to be, you'll spawn in the same world as last time. You will NOT have all your items, though.) Mods are typically updated on an irregular schedule shortly after the game itself is updated (though this depends on how quickly the mod author or maintainer gets to it), so there will always be a period following a new release when no mods are safe to use yet.

Mods that only alter the behavior of existing items, blocks, or mobs are fairly safe; they should typically be able to be installed or uninstalled at will, without corrupting your save (although if you have structures that rely on behavior introduced by a mod, then they won't work without the mod installed). Note that this includes mods that add crafting recipes without adding new items/blocks.

Mods that introduce new items, blocks, or mobs are not safe to uninstall. Once you have saved a new item/block/mob into your world, you must never play that world without having the mod installed, or you risk crashes or corruption. (Should you wish to uninstall the mod, you must either abandon your previous saves that used it, or ensure that you destroy all of the new items/blocks/mobs in your world before removing the mod. Or you can use a program such as MCEdit to remove the block after removing the mod, which renders your save file usable again. For corrupted inventories use INVedit) Also remember that updating the client counts as uninstalling the mod (unless you immediately reinstall the updated version of the mod).

Mods that change how chunks are saved will render saved worlds incompatible with 3rd party software not designed to work with them. An example of such tools are map rendering tools, such as Cartograph G, that rely on parsing save files to produce their end results.

Multiple mods may conflict with each other; certainly if two mods need to alter the same files, then they are unlikely to be compatible.

If you wish to install a mod or mods, and want more detailed information on the process, please see the page: Installing Mods.

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
This list only contains entries that cannot contribute to griefing or cheating in multiplayer.

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

Merged Mods
The Aether Combines a map generator and a mod to create the Aether. Under Progress

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.

Beta Server Wrappers
There is a full list of server wrappers at the Server Wrapper page