There are a bunch of wallpaper apps available for the iPhone that offer hundreds and thousands of wallpapers. Unfortunately I've found most of them to contain wallpapers that aren't of a certain quality - they're pretty generic.
Wallpapr is a new iPhone wallpaper app that offers a select few wallpapers which are of very high quality. The owner of the app prides himself in choosing only the best wallpapers and giving users the ability to browse them in a very clean and flat interface.
It's free and worth a look at. The wallpapers on there are pretty unique and aimed very well at designers.