Earlier this year, Northwest Airlines announced that they would be offering E-Boarding passes to select cities offering nonstop flights.
Wondering how it works? When you check in to your flight on nwa.com, just select the E-Boarding Pass option from the drop down on the "Completing Your Check-In" page. From this page, you will be prompted to enter your cell phone information, and your E-Boarding Pass will then be sent to your phone. Once you get to the gate, you will have to show your screen with the 2D barcode and scan it right there.
So far, people have reported that it works great — minus the TSA part since they usually check your paper boarding pass with your ID as you walk through the security X-Ray Machine — something they'll have to work on improving.