I prefer the inverter option. It lets me run soldering irons and smallish power tools off my car battery as well as my laptop. I have a 750w one but I've seen up to 1500w. I've also got travel adapters for my Dell and Lenovo corporate laptops that come with 12v cigarette lighter adapters. Not sure if Dell has these for their retail laptops or netbooks.
Another option if you're not near your car is to mount a small battery in your tool box. A sealed NS40 or NS60 sized battery should fit in most medium to large tool boxes. My toolbox (http://www.amazon.com/Plano-Molding-.../dp/B000CRBHYE
) is on wheels so weight is not too much of a problem and I've been considering mounting an NS40 size in there for use at the track. Would just need to be recharged manually when I get back from the track.