From Blender's website:

• http://www.tellim.com/texture_cd
• http://www.imageafter.com/
• http://www.digitalcraftsman.com/textureBin/textureBin.htm
• http://www.vb3d.com/Textures.html
• http://www.3drender.com/light/EqTutorial/tiling.htm
• http://www.mayang.com/textures/
• http://gw.marketingden.com/planets/planets.html
• http://www.3d-rekonstruktionen.de/downloads/
• http://www.mega-tex.nl/

And some Blender material libraries:

• http://www.free-webspace.biz/sonix/Cars/Blender234CarMaterialLibraryR1.html
• http://www.tellim.com/random/matlib_v102.blend
• http://www.elysiun.com/forum/viewtopic.php?t=57206
• http://cs.unm.edu/~sketch/gallery/resource/furlib.html