After browsing around the web, I found many sites that collect all nice and cool wallpapers. And among all of those, my favorite wallpaper pools are:
Wallpapers of wonderful panorama
These wallpapers I got from InterfaceLift. There are much more of those cool wallpapers inside this site. Gotta go to check it.
Wallpapers of amazingly beautiful nature
I simply love the beauty of nature and KDE-Look perfectly provides this kind of wallpapers. Lucky for me (and maybe for you).
Everyday Apple. If you are so in love with Apple, you will prove that by attaching one of these wallpaper in your Mac. The Apple Collection proved to be really in love with Apple (just my opinion).