Dropbox (iOS | Android) is a free and extremely easy-to-use tool for sharing files, photos, and videos, and syncing them among your devices. You can also use Dropbox to back up files and access them from other computers and devices (including smartphones and tablets), with dedicated apps for each device you own.
The Dropbox interface makes it easy to share or sync files (pictures) See full gallery
Dropbox's interface is incredibly intuitive, which is part of what makes this app such an obvious choice for syncing files. When you install Dropbox on your iPhone or Android device, you'll have immediate access to your uploaded files. You can also access the folder from other computers or smartphones, or anywhere you have access to a Web browser. Even better, you can use the app to share files with other people -- either through a designated "Public" folder (for everyone), or by setting up a shared folder for specific users.
On Android, buttons for navigation are across the top, while on iOS they reside at the bottom, but there's no difference in usability. Your Dropbox folder is the main screen, but you also have buttons to quickly view your photos folder (and enable auto-uploading); look at Favorited files (the best place to keep things for quick access); and a settings section where you can review your account info. The app settings also let you upgrade your account and add a passcode lock to Dropbox if you need added security.