I am not personally a huge advocate of joining the WiFi network onboard planes. There's a certain tranquility for me in that 6-miles above the earth, for my hour or two in the air, my phone won't ring or buzz with an e-mail I typically use the time to catch up on reading I've fallen behind on or just think through the problem of the day uninterrupted. But I do occasionally succumb to the need to fire off some e-mails, especially if the flight is long enough. This month you won't have to pay for the privilege if flying Delta.
Delta Air Lines has teamed with Diet Coke to offer travellers free WiFi on all flights during the month of August. The code you'll need is "DIETCOKEGOGO". I haven't tried it personally having been uncharacteristically grounded for the last couple of weeks. I've heard it is working though, and I will give it a whirl on a few flights in the week upcoming.