Tutorials/Programs and editors

The Minecraft community has created dozens of programs that can be used to enhance the game. Server software is so numerous that it has a page of its own. The most popular programs are Mappers that create an overhead view of a level. Map Editors let you directly alter a level outside of the game. Level Generators create new terrain types and block patterns that aren't available normally. Mods and Total Conversions add completely new features or modes of play to the core Minecraft game. Backup Utilities make it easy to play your favorite worlds on mulitple computers as well as giving you peace of mind knowing that your worlds are safe. Finally, there are Specialized Programs for editing your inventory, simulating Redstone circuits, and altering game files at a low level.

Software is Windows-only if not specified.

Mappers
Overhead mappers create a literal map of your level's terrain. Many mappers have options for different overhead and angled modes, and some will limit the map to selected block types or slice the map into smaller pieces.

Minecraft Overviewer
A Google Maps powered overview that draws your map in 30 degree isometric mode with a high level of detail. Source code and demos are available on github.

OpenCraft Plotter
From the OpenCraft project, comes Plotter. It is a java library and standalone tool designed to have an expansive suite of visualizations for every known format of Minecraft save file.

Map Viewers
Map viewers are read-only and interactive.

Map Editors
Map editors alter a level's terrain or other properties, such as player position and time of day. Some of them also let you edit your inventory and generate terrain.

Omen
The first map editor, created to edit Classic levels. With it, you can view the level's blocks in 2d slices, and alter them by "painting" blocks using brushes of different shapes and sizes. Also includes some terrain generators and supports the old Indev levels. Requires minecraft-server.jar for Classic support.

Created using Java and reportedly works on Windows, Mac, and Linux.

Forum thread. Classic only.

NBT Forge
A successor to Omen that supports the newer Alpha levels. Like Omen, you can view 2d slices and paint blocks using a different set of brushes. NBTForge was initially created for the old Indev format.

Forum thread. Alpha only.

MCEdit
The first 3D editor. Created to import older levels into the new Alpha levels. Fly around in 3D and select blocks, then copy or export them. Import blocks from schematics or from entire Classic levels. Resize a level by creating or deleting chunks.

Forum thread.

MineEdit
All-in-one Editor. Includes a terrain editor similar to Omen and NBT Forge. Replaces blocks of one type with another. Also lets you edit your player's inventory and the world's entities.

Created using .NET.

Forum thread. Alpha only.

LolEditor
View the terrain with different viewing modes and show block statistics. Edit entities and inventory, and create randomized items.

Created using .NET.

Forum thread. Alpha only.

MapDeleter
Resize a level by deleting chunks. Copy and paste chunks from a top-down view, or add tall pine trees to a level.

Created using Java. Reportedly works on Windows, Mac, and Linux.

Forum thread. Alpha only.

mce
Command-line editor derived from MCEdit. Works well with servers.

Forum thread.

Level Generators
These programs will either create a new level with custom terrain, or "implant" new blocks into an existing level.

ZOMG
Creates wild, surreal landscapes of several varieties. Forum thread. Classic only.

MCMapper
A dungeon generator seemingly inspired by old Roguelike games. Creates rooms and levels out of different materials, in different styles. Rooms may hold traps, statues, enemy spawns, and/or treasure. Forum thread. Alpha only.

DF2MC
Converts a Dwarf Fortress level to a Minecraft Alpha world according to a ruleset.Forum thread. Alpha only.

Portal
Converts Minecraft Classic into a Portal-like game with working portals, buttons, and storage cubes. Includes a texture pack and custom server software.

Forum thread.

Custom server software for starting your own Portal server.

Runecraft
Adds magical runes to Alpha Multiplayer. Teleport to different places and enchant your weapons by building runes on the ground.

Forum thread.

MSM
Minecraft Save Manager is a small launcher tool that will back up and synchronize your saved games across multiple computers, performing file operations both before and after Minecraft is launched. It's meant to be a zero-effort tool so you don't have to go through the pain of loading your saved games from a  USB drive or cloud folder. This program is in Beta phase and lacks error handling and many other features seen in MAU. Forum Thread.

MAU
Minecraft Assist Utility currently features a game launcher and robust back up system that is both easy to configure and painless to use! Designed for use with multiple computers or on a single PC! Features not seen in MSM: configuration Gui, proper error handling, unique time saving back up system, game launcher supports multiple browsers as well as the standalone, full user control over restoring files(no automation no mistakes). Official site. Only tested with Alpha

TreasureChest
TreasureChest is an easy to use MC backup utility that will help you to backup and restore saved games. It can manage multiple backups and is capable of repairing/replacing chunk errors or corrupted savegames. TreasureChests is available for Windows (XP/Vista/7) and Mac OS X and updated frequently. Forum Thread. Alpha only.

INVEdit
An inventory editor that lets you add and remove items. Also exports your entire inventory, to copy between levels or use as a preset. Forum thread. Alpha only.

Redstone Simulator
A simulator for Redstone circuits. Place circuit elements in a grid and use Play/Pause to control the simulation. Forum thread. Alpha only.

NBTEdit
Development tool for working with the NBT format used by different level files. Can be used to edit inventory, entities, and world attributes. Forum thread. Alpha only.

Minecraft Canary
Android app that serves as a full guide to Minecraft. Features a Timer that tells you what time of the day is it. Contains a full recipe list, tips, info about mobs, block IDs.. Coming to iPhone soon. Official site

Minecraft Handbook
Android app for everything Minecraft. Good reference guide. Contains a nice recipe viewer. Official site

Sk1n Sw1tch
Easily switch between different graphical packs. Forum thread. Alpha only.

Minecraft ID List
An interactive ID list that allows you to search through blocks and items. Alpha only.

Minecraft Texture switcher
A Windows only program which allow you to switch textures of Minecraft, or even add your own. Wiki Page

(Finished texture packs can be found here: Forum Thread.)

Server Wrappers
Server wrappers are mods that do not modify the Minecraft server or its data files, but instead read its console output and supply console commands in order to add additional functionality.

McMyAdmin
McMyAdmin is a web based administration tool supporting both Windows and GNU/Linux servers.