There are lots of free textures here at Renderosity.  http://www.renderosity.com/mod/freestuff/index.php?section_id=3

Another good site for free stuff is 3Dcommune.  Site requires free registration.
http://www.3dcommune.com/3d/free.mv?Vue_Free_Stuff+tms+1226436791

And of course, GeekatPlay has a lot of free stuff, but will also teach you how to make your own materials.
http://geekatplay.com/index.php

Then there are all of the Renderosity people - a very generous group who will supply you with many links.

LVS - Where Learning is Fun!  
http://www.lvsonline.com/index.html