File Formats don't matter. Please do not add commercial 3D model sites, unless they offer a huge collection of free models aswell.
Textures
http://www.imageafter.com/textures.php
http://developer.nvidia.com/object/IO_TTVol_01.html
http://www.freefoto.com/browse/33-00-0?ffid=33-00-0
http://www.noctua-graphics.de/english/freetex_e.htm
http://www.grsites.com/textures/
http://www.absolutecross.com/graphics/textures/
http://www.m3corp.com/a/download/3d_textures/pages/index.htm
http://studio.planethalflife.gamespy.com/textures.asp
http://local.wasp.uwa.edu.au/~pbourke/texture_colour/index.html
Models
http://www.highend3d.com/downloads/
Sites that have both
Tools
Generates Explosion Sprite sheets http://www.geocities.com/starlinesinc/
Procedural Materials http://www.spiralgraphics.biz/viewer/
Reference Images, Concept Art and the likes
http://www.prideout.net/colors.php
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