As the only mass-produced passenger car on the market that's powered entirely by electricity—with no engine, no fuel tank, and no tailpipe—the 2011 Nissan Leaf is arguably the cleanest, greenest car on the market, with the lowest carbon footprint, if you're looking only at energy used. Unlike the 2011 Chevrolet Volt, which only goes about 40 miles then runs on gasoline, with the Leaf you have to closely watch how far you drive on a charge.
Electricity for charging costs far less than gasoline, but how much greener electricity is depends on where you charge up. If your electricity comes from the oldest, dirtiest coal plants, then it's not a solid step forward; but if it's from wind farms and nuclear plants, then you're probably doing your environment a favor.
The Leaf takes about 14 hours to charge with a 220-volt charger, possible around heavy-duty appliance outlets.