It's green month here at CNET Car Tech. That means a focus on alternative-fuel vehicles and emissions-reducing technologies, as well as a peek at how the automotive world will look in the post-petroleum age. Here in the present, a number of car makers are already doing a pretty good job of making low-polluting, fuel-efficient models. Each year, the American Council for an Energy-Efficient Economy (ACEEE) puts out its list of the 12 "greenest cars," taking into account tailpipe emissions, gas mileage, and curb weight. Here's our round-up of this year's least dirty dozen.