Tes3Mod:Construction Set

thumb|right|300px|TES Construction Setthumb|right|The "ninja monkey" model representing a leveled creature in the CS
The Elder Scrolls Construction Set is the same one as the developers at Bethesda used to produce almost all of the game's content. Features include:
It can edit almost all aspects of the game excluding 3D object creation. Unofficial 3D Studio MAX, Maya, and Blender plugins have been released to allow people to make their own models.
It is designed to allow easy editing while providing extended operation for experts to create more complex objects.
All changes, additions, modifications, or deletions by the editor are stored in files called plugins. Bethesda says that a typical plugin (small dungeon, loot, quests, etc...) should be around 200kb in size. Additional models, sound, or music may increase that size significantly though.
There is effectively no limit to the number of plugins you can apply to the game. This means that you can download and install whichever plugins suit your fancy. The game has good checks in place to ensure that multiple plugins do not interfere with each other.
Plugins can easily be added or removed from the game. Thus, if you install a plugin which messes up the game or you don't like, it can be removed.
Typical creation time for a decent plugin is very small. Developers at Bethesda say they were making simple quests and dungeons in under an hour when they first started to use it. The editor uses a drag-and-drop technique to accomplish this. For example, you might drag several dungeon sections from the library to create a full dungeon, and then drag in a few monsters and NPCs and some items/treasure for atmosphere.
Simple scripts (no function definitions) can be implemented for most objects in the game allowing for practically limitless possibilities.
The Construction Set is available for digital download from  . It also comes bundled with most versions of the game.
The free Bethesda.Net edition does not include the Construction Set, nor does the Steam version of the game.

Quick Tips
Keyboard Shortcuts - Useful list of the CS keyboard shortcuts.
General Editor Tips - Some short tips to help get you started.

Cells
Editing Interiors - Editing interior cells.

Cleaning
Creating Clean Mods - How to make sure your mods are 'clean' and avoid the dirty save bug.

Dialogue
Dialogue Notes - General tips to editing dialogue in the CS.
Dialogue Avoiding Greeting Conflicts - Specifically deals with the common problem of greeting conflicts in dialogues.

Scripts
Alphabetical Function List - All scripting functions, sorted alphabetically.
Categorical Function List - All scripting functions, sorted by category.
Scripting Basics - An introduction to scripting.
Scripting Pitfalls - Common and unexpected scripting problems and how to deal with them.
External Links:
Yacoby's Morrowind Scripting for Dummies 9.0
Additional notes on Morrowind Scripting for Dummies 9.0
Scripting Tips - John Moonsugar offers a few notes regarding scripts. (CellChanged is fixed by the Morrowind Code Patch v2.3)

Tutorials
Morrowind Modding Wiki thread @ Official Forums
Modding for smarties - abot offers his experience and best practices for the serious modder.
Morrowind Mod Maker's Bible (.pdf)
Tamriel Rebuilt Tutorials

Archived links:
Tutorials and more by Scott Fisher
Tommy Khajiit's Tutorials and guides
Dave Foster's series of tutorials on the CS
Comprehensive Tutorial and Useful Link List @ Official Forums