http://www.photoshopcafe.com
^We use this in my intro to digital media for some of the simpler tutorials
A few of the tutorials I particularly like:
http://photoshopcafe.com/tutorials/Brushed_Metal/brushed.htm
^brushed metal texture
http://photoshopcafe.com/tutorials/rock/rock.htm
^Crazy easy rock texture
http://photoshopcafe.com/tutorials/sketch/sketch.htm
^Dunno if this can easily be applied to game art, but it looks so cool
[EDIT]
http://www.photoshop-garden.com
^Stumbled upon it
http://www.photoshop-garden.com/view_tut.php?id=11
^Neat semi-realistic looking fire.