Southwest this week began testing their own in-flight wi-fi service, based on a satellite connection from Row 44. It's on one plane now with more coming soon, and right now, it's free.
Southwest is using a satellite-based system from Row 44, and not the cellular connection used by most other airlines with Aircell's GoGo service. So if you find yourself on that one (and soon to be handful) of Row 44-equipped planes, you can surf for free. [USA Today]