These kind of link-lists have a habit of falling out of date quickly. Not sure what to do about that. But they are helpful.
Post your favorites, try to avoid duplicates, blah, blah, you know the story.
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Here are some texture sites that offer no restrictions on commercial use, a very important disctinction for me.
http://www.planetunreal.com/undulation/textures_n.htm
http://www.1000skies.com/fullpanos/index.htm
http://home.zonnet.nl/textures01/index.htm
http://www.animax.it
http://www.monitorstudios.com/bcloward/resources_textures.html
http://www.frenchtextures.com/site_gr.html
http://www.graysage.com/cg/BackDrops/index.html
http://www.imageafter.com/index.php
http://perso.club-internet.fr/lemog/lemog_textures/acc_textures01.html#
http://www.mayang.com/textures/
http://noctua-graphics.de/english/freetex_e.htm
http://perso.club-internet.fr/sebastien.gaucher/textures/textures.htm
http://earthobservatory.nasa.gov/Newsroom/BlueMarble/
http://visibleearth.nasa.gov/
[edit… fixed bad links]