Nature
http://blogs.msdn.com/mswanson/articles/wallpaper.aspx
http://www.graffitiwallpaper.com/index.php?size=6
http://co-opgaming.com/nowai/ (Random: Hillary Clinton!?)
http://www.geneburch.com/
http://nowai.wordpress.com/tag/wallpapers/
http://www.crestock.com
http://www.coldforged.com
http://controltheweb.blogspot.com/ Verticals
http://www.wpdb.de/index.php

Space
Top Ten of 2006 (most higher res, but I'm not complaining)
http://athena.cornell.edu/gallery/wallpaper.html
Mad Cow Disease: Space