Free, high resolution (1600x1200) textures and reference images
A Photoshop tutorial on creating seamless repeating texture maps from photos
Free Digital downloadable textures for 3d builders and skinners for every 3d building
A collection of fur and other hair presets by BrianH (old elysiun topic 57206).
The name is a bit misleading as it includes particle based presets for fur, but also feathers, grass or even plastic like strands.
Different paints, stones, metals, glasses and even microscope fx.
Meta Material Library by [Meta-Androcto] with over 400 materials.
Online and user supported material archive
Here you will find Blender files that use different techniques to create special effects. Effects ranging from smoke to water
A texture plugin to render colors of the sky as lit by the sun