The Power of the High Pass Filter
May 2001 - Peter Hajba
This article discusses some very good techniques for texture artists.
October 2001 - Mark Peasley
This article shows how to create a basic tileset by creating a flood-fill texture that is tileable not only in one orientation but however you rotate it. From this flood-fill texture, three other textures are created for transitions into other material.
Adobe Studio Exchange
Exchange photoshop files, such as actions, brushes, etc.
This place don't offer free material. But the images and animations are of very high quality so it might be worth the price.
A great place to go for free textures, 3d models, and audio.
This site holds plenty of great textures, and more is added every week. A licence must be purchased in order to use all of the textures, but I believe a small selection of textures are offered for free every week.
A large collection of quality textures. The only problem is that the keeper don't know anymore which ones are free to use. But using the textures as reference when creating your own should not be a problem.
This is a custom search engine where users can upload game resources and let other users download them for free, or for a price.
This site has plenty of excellent textures that can be used in games, and mods. It also has a couple of tutorials for how to make your own textures.
This site has a lot of high quality textures that can be used for free in non-comercial products.
This site holds lots of free stuff, like textures, sounds, fonts, etc.
Texture Maps of Earth and Planets
Here are some very good texture maps of Earth, and other planets.
Sky's The Limit
This site offers several sky cubes for a very cheap price, there are also a couple of free ones.
Mayang's Free Textures
A very large collection of free textures. The textures are not seamless though.
NASA/ESA Hubble Space Telescope
Here are many excellent images from outerspace taken by Hubble.
A free utility for creating seamlessly repeating wood textures.
Free textures, even for commercial use.
Both free and commercial textures and 3D models.
Paul Bourke's Textures
Lots of textures that can be freely used, at least for non-commercial products.
How in the World Terrain Textures
A large texture package of what looks to be high quality textures for various types of landscapes as seen from high above. Perfect for someone making a flight simulator. The price is good too.
Quality textures and images for free. If the premium member ship is purchased you get even higher quality textures and bump maps to go with them.
Lost Garden: 250 free handdrawn textures
Some cool looking tile textures that were made for a game that never saw the light of the day. The author released them for free.
The Planetary Classification List
Imagery and information about planets. Suitable for inspiration.
Free sky boxes
Beautifully rendered sky boxes with canyons and volumetric clouds.
An image database of cool images suitable for wallpapers. It's also a great source of reference material.
The Universe in Color
A series of images of various nebulae and galaxies. Perfect as reference material for those space environments you want to create.
Genetica Texture Packs
Free tiling textures
Free textures from Bug? Galore!
Few but good free tiling textures with separate bumpmaps.
Free stock photos, that serve as an excellent base for game textures.
A PhotoShop plugin for generating seamlessly tiling filters. The site also has a large library of procedurally textures for viewing.
An awkward interface to rather large collection of photos useable to create game textures.
oZone3D Texture Pack
Free pack of tileable textures.
Royalty free high quality textures for download. Looks like a low subscription fee is required to be able to download.