oswd.org
Open Source templates. Free for you to use, as long as you leave the credits in place.
http://www.oswd.org

pixelgirlpresents.com
All the desktops and wallpapers you could ever wish for. Be sure to check it out!
http://www.pixelgirlpresents.com/desktops.php

sxc.hu
Here you will find free stock pictures. If it is not here then you will not find it!
http://www.sxc.hu/