Lots of links to tuns of textures.
http://www.3dlinks.com/textures_free.cfm
Another great texture pack for mapping and modding ( 124 meg. )
http://www.planetquake.com/berneyboy/
Some good sky backgrounds and some "fish-eyed" photos of skies. Great for making a half-sphere then applying your texture directly to the top, then placing the dome over your scene.
http://www.philohome.com/gallery.htm
Good sky backgrounds ( non-panoramic )
http://geekphilosopher.com/MainPage/bkgSky.htm
Other good sky backrounds ( Panoramic )
http://daemon.realsoft.com/Environments.htm
seemless sky backgrounds
http://www.noctua-graphics.de/english/Tex/sky.htm
other good dome sky ( fish-eyed ) backgrounds
http://www.philohome.com/skycollec/skycollec.htm