United Airlines is introducing a new "trial feature" to its app that lets international travelers scan their passports along with check-in using their phone. The app can use the scanned image of your passport to either verify previously-entered information or add it to the system, removing one step that slows down the ticketing and boarding process of international flights. If the app properly identifies your passport information, you won't be asked to provide any further documentation to receive your boarding pass.
Naturally you'll still be expected to bring valid passports and identification with them to the airport, but with this latest app update you'll likely get to leave that passport in your bag and breeze on through the gate to your flight.
You can grab the latest version of the United Airlines app from the Google Play Store link at the top of this post.