The power port on United flights are similiar to the cigarette plug in your car. Not all United planes have the ports. American Airlines have the power port available on most of their flights. Check seatguru_com for the which airlines and which planes are equipped with them. These use to be only available to First and Business Classes, but it's widely changing. Unfortunately, many of the cheaper airlines (i.e. Southwest) don't have the power ports. And yes, there's no charge to use the ports.
The Eddie Bauer portable DVD players (PE702 & PE802) both come with cigeratte lighter adapters.
Here's one review on the PE702:
Pros: Stylish, very portable, very good battery life; decent sound from the speakers
Cons: Poor flesh tones and deep reds/blues. I've seen better on Toshibas and Zenith models
I returned my first portable player to Target (don't even remember the no-name brand) in favor of the Eddie Bauer (Audiovox) PE702. This little player has a lot going for it. Its sleek exterior complements the overall video and sound quality of the machine. Hook it up to some computer speakers with a subwoofer and you'll think you're in a surround sound movie theater. The sharpness of the display is very good; however, you must be angled perfectly to get the full benefit of its color capabilities and across the screen full color range.
Another nice feature is the "screen mode" button that switches between wide screen and standard (older) formats.
The only complaint I have are the weak reds and tepid flesh tones especially in comparison to a regular television monitor.
Battery life seems to be as advertised...about 2 hours and 45 minutes (give or take).