Making the Movie notes free texture resources and adds some textures of its own. More free textures can be found at David Torno's MacTex (see his recent Index post).
Ever popular Earth textures (various strings attached) can be found at JHT's Planetary Pixel Emporium (with C4D tutorials), Unearthed Outdoors True Marble-free version, and NASA's Blue Marble Next Generation. Tutorials for earth maps and textures were mentioned in a previous post, 3D Earths.
Of course there are now a ton of other free textures available on the web. One of the better resources that stepped beyond to layering was the Alcatraz-inspired "Surface of Reality" CDs from Alex Lindsay, which later morphed into the Surface Toolkit and expanded in several directions in the dvGarage Surfacing Series. The Grunge Tutorial by Harry Frank shows one way to use these type of textures in AE.
Basics for understanding the imitation of the surfaces of ephemeral reality can be found in "Digital materials and virtual weathering," Scientific American, February 2000 (PDF). Fun books on materials can be found at William Stout in San Francisco and Builders Booksource in Berkeley.
Update: Recycling an item on 3D surfaces from New Scientist:
Update: More than 60 Impressive Free High Resolution Textures and Backgrounds can be had though Photoshop Roadmaps.