cgtextures

cgtextures is a texture website that offers thousands of free textures.
Marcel Vijfwinkel, being a 3D artist himself, knows how difficult it is to create textures without good photo materials. Users can download an amount of 15 MB every day after registering for the free account.

The site also offers some great tutorials for working with textures and making them look better.

Texture-categories include:
Animals, Bones, Brick, Buildings, Concrete, Decals, Doors, Fabric, Fire, Food, Ground, Grunge, Ink, Landscapes, Manmade, Marble, Metal, Nature, Ornaments, Paper, Plaster, Plastic, Roads, Rock, Roofing, Rust, Scrap, Signs, Skies 360, Skies Partial, Soil, Splatter, Tiles, Various, Water, Windows, Wood, Wrinkles, X-Rays

More info on www.cgtextures.com