Platforms: iOS, Android
Dropbox is an incredibly useful app that solves the problem of getting files from your computer onto your phone and vice versa. One of the very first multitasking apps, it also lets you easily transfer files between computers because it uses cloud storage -- the data isn't stored on a hard drive, but in an online network.
After installing Dropbox on your computer, any file you save to your Dropbox will automatically sync with other computers or devices with this app on it. For example, you can take pictures with your iPhone, move them to Dropbox and they'll show up on your home computer. They will also sync up with the Dropbox Web site. After you use this app for a while, you'll wonder how you ever lived without it.