On Delta and A"A flights that have GoGo wireless, you can use the codes:
DELTATRYGOGO
AATRYGOGO or 1939admiral70aa
These codes will only work once per username, but lucky for us, GoGo lets you sign up very easily with no requirement for a valid e-mail address (just type in a fake one) and no need for a credit card. So every time you are on a flight, just sign up for a new account and use the promo code.
I use this one every flight and have been enjoying plenty of free WIFI. I'm posting this on my EWR-ORD flight right now. Sorry, gotta go...we're landing in 10 minutes.