My family and I are in the process of going paperless and we recently had a good experience with the Southwest boarding pass. They let us checkin online and sent all of our boarding passes to our wireless devices. It was great as all we had to do was go to a kiosk at the airport to get our boarding passes printed up. As a bonus, they also sent us mobile updates letting us know what was happening with our flight.
I do wonder though, when you'll be able to not have to print anything and just be able to show your e-mailed barcode to get on a flight? That would really be something.