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.
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.
Dropbox is especially good for backing up your files online, although the biggest barriers to this are the size of your backups. You get 2GB free with Dropbox, or you can choose 100GB, 200GB, or 500GB with a monthly fee. There are also business plans that start at 1TB for five users. You'll just have to make sure that the files you want backed up live in the Dropbox folder.
The only issue I have with Dropbox is the somewhat limited amount of space at 2GB, but by completing various actions, I was able to get more. You can get up to 18GB by completing tasks such as sharing the app with a friend, adding photos for the first time, and sharing a folder with multiple people. Yes, it provides incentive to learn the various features in the app, but I still think the lower limit should be higher to begin with. It's hard to criticize Dropbox when the app is free, but I would have liked a higher beginning limit (such as 5GB).
Some recent additions to the app add more features for sharing what's in your Dropbox. You can now share, delete, or move a file faster by tapping it in Android or swiping in iOS to bring up quick menu icons for completing each action. In the photos section, you can now select multiple images for sharing with friends or family. Each update to the app seems to make it even easier to use.
Overall, Dropbox gives you a lot of useful features for free, with a remarkably smooth interface and a shallow learning curve. If you find yourself frequently transferring files between different people and computers by hand (either via e-mail or USB drives), Dropbox is a must-download.