There's nothing like free or cheap in-flight Wi-Fi, and in-flight Wi-Fi doesn't come cheap.
Here is one hack I wish I had thought of, and one I have actually used. Some of these basically depend on the specific airline you're flying on, but it just goes to show there is more than one way to think outside the box.
Gogo dominates the field for in-flight Wi-Fi on the domestic airlines (and it's really best to buy it *before* flying), but if you're going to bite the bullet at the last minute, the best way to do it is to get the cheapest 30-minute pass. Once I'm online, I'll ping someone on Gchat with my Gogo details, and ask them to buy a day pass under my account. Once time's up, I'll log back in all normal like. Beats paying $39.95.
On the way further afield? Quora user Anirudh Holla has a great tip if the airline is Emirates:
Emirates would give free internet for 30 minutess on all international flights from Dubai to Europe, North America etc. Use the free internet for first 30 minutes, after 30 minutes open incognito window and register with a different email id. I noticed they don't check email ids. So anything in the form of [email protected] would work.