Local-only storage is so 2008. With FolderSync, you can let your Android device effectively live in the cloud: The app lets you set up advanced sync rules to keep your phone or tablet connected to any number of cloud services. That means the data on your device is always backed up and available anywhere you go.
FolderSync works with a large number of services, including Dropbox, SkyDrive, SugarSync, Box.net, Google Drive, Amazon S3, and even your own personal server space via FTP or WebDAV. It can sync files one- or two-way and can do it either on a regular schedule or instantly anytime changes are detected.
The app costs $2.95; a limited-feature and ad-supported version is also available for free.