http://www.clearbits.net/
Searching for 'wallpaper' has a few collections. and CC images from flickr http://flickrcc.bluemountains.net/flickrCC/index.php?terms=wallpaper&page=1Also, searching for generic terms like 'nature' , 'beach' or 'desktop' will show a few suitable ones.
extra: I keep these on an online bookmark site for easy personal reference, at http://perthfreeculture.org/