Sites worth visiting:

- General Links -
Through The Looking Glass Forums
The official TTLG forums. The best place to go to ask questions and find answers from the Thief community.

The Circle of Stone and Shadow
The official TTLG Thief site, and the first place to go for Thief stuff.

Thief Underground
Another good Thief website.

The Keep Of Metal And Gold
An excellent place to go for Fan Missions, even if the non-profit FM CDs/DVDs service is no longer available.
The Keep also hosts small FM-building contests every few months.

Thiefmissions
This website has nearly every Thief Fan Mission availiable for download.

- 3D Models -
The Low Poly Guild
A website dedicated entirely to custom objects for Thief .
The first place to go if you're looking for 3D objects, modeling tools, and tutorials.
There are some Morrowind objects to be found there, too.

Studio of Schwaa
One of the best places to go if you're looking for a custom object or AI mesh.
There are also some tutorials on how to make your own 3D models, alongside info and screenshots from Schwaa's excellent missions.

Shadowspawn's Thief Pages
Here, you can get all of Shadowspawn's amazing utilities, the re-jointed burrick mesh, and more.

Model Maker's Vault
A website that was planned to become a depot for all your 3D object needs.
Unfortunately, it hasn't been updated in well over a year.

Dark Arrow's Thief 2 Site
Not only does this page have some nice 3D models, it also has some interesting demos, tutorials, and missions in varying stages of completion.

The Watcher's Site
There are a few nice objects to be found here.

Vertex Lighting Tutorial
Some useful information about vertex lighting, which the Dark Engine uses to light its objects.
Although this tutorial is written primarily for the Unreal Engine, most of the information applies to Thief too.

- Textures -
The Nameless Tower's Texture Vault
Here you can find several texture familes made by me, and also some texture tutorials.

Cheap Thief Textures
Here you can find several texture families, already sized and palettised for Thief 2.

Lemog 3D Textures
A nice collection of seamless textures that can be freely used for whatever purpose.
Textures include wood, bark, tiles, skies, water, frost, metal, bricks, snow, wallpaper, and rugs.
You will need to scale and palettise the textures yourself, though.

MoG's Textures
Several nice texture families can be discovered here. They are all palettised and ready for Thief 2.

David Gurrea's Textures
There are some really nice high-resolution textures to be found here, all of them ideal for a medieveal setting.

Mayang's Free Textures
A large collection of image that can be freely used for whatever purpose.
In most cases, they will need as bit of work to make them wrap properly, though.

ImageAfter
Another huge collection of royalty-free images.
Again, it'll take a bit of work to get them to wrap properly.

Balatro's Texture Packs
Three packs of various Thief textures.
Unfortunately, they were made for Thief 1 / Gold, and therefore they all use the default palette (rather than an optimised one), so the quality is not perfect, but there are still some nice textures to be had.

Schwaa's Textures
Some textures.

R Soul's textures
Some vegetation textures.

- Tools -
Texture Maker
A very useful program that can allow you to easily create and modify textures, using a collection of blending and generation tools.
The version cannot save images larger than 320x320 pixels, but that's more than adequate for Thief 2's 256x256 maximum.

Thumbnail Viewer
A free, open-source utility that adds the ability to preview additional image types in Windows' thumbnail view, inclduing .pcx, .tga, and nVIDIA's .dds format.
A pity that it only works in 32-bit colour (and not 16-bit mode.)

- Other Resources -
Olga's Gallery
A vast art gallery where you can find paintings by many famous artists throughout real-life history.
I would assume that anything painted by someone who died 400 years ago is out of copyright, and can be freely used in a Fan Mission.