There are a lot of great wallpaper sites out there, but Desktoppr is a little different. With a focus on only beautiful, high-resolution wallpapers, it uses Dropbox to download and upload the wallpapers you like, so your desktop always has a fresh look without a lot of work involved.
Desktoppr is simple: just sign up, connect it to your Dropbox account, and start browsing wallpapers. There aren't a lot of search options; you mostly just search by resolution, but don't let that scare you off -- even if you have a specific style, you'll be blown away by each and every wallpaper on this site. They're all gorgeous and high resolution (including even a section of over 700 retina-ready wallpapers for the new MacBook Pro), so you really can't go wrong. Just find a wallpaper you like, click the "Sync to Dropbox" button, and set your computer to watch your Dropbox folder for wallpaper images. Whenever you add a new one from Desktoppr, it'll be added to your cycle, and if you want to upload one to Desktoppr, you just stick it in your Dropbox folder as well. It'll be added to the Desktoppr library within a few days.
Desktoppr is in beta right now and you'll need an invite in order to use it. Sign up for them at the site below.