Android: There are a lot of file-sharing apps to choose from in the Google Play store. But this is the first we've seen that can create an ad-hoc network for sharing files between devices if no Wi-Fi network is present.
You'll need SuperBeam installed on both devices, and then you'll need to pair the devices using QR Codes, NFC or by typing a manual code. After you've set it up, sharing is as easy as picking the files you want to share and sending them to the paired device.
You can even connect Superbeam to an Android device without Superbeam installed or to a computer by connecting it to the ad-hoc network Superbeam creates.