Dropbox is a cloud storage and sync service available for the Mac, PC, and most mobile devices. It makes sure that anything in its special folder is kept safe online and mirrored on any other device you install it on. What's more, Dropbox provides a powerful API for developers that allow many other apps to tie into their service and use them for cloud storage and sync as well.

Many apps use Dropbox to store files and settings. For example, several mail apps for both Mac and iOS use Dropbox to safely store attachments or user preferences. Some examples of this are the Mailbox, which is now owned by Dropbox, and apps like 1Password which can sync your keychain to all devices via Dropbox.