The American Council for an Energy Efficient Economy has named their most environmentally friendly vehicles for 2008.  Members of this company include:

HONDA CIVIC GX: Powered by natural gas; appearing all-natural Civic; and achieving a 36mpg equivalent, this Honda is especially CO2 friendly.  If such a thing exists.
TOYOTA PRIUS: I’m sure you’ve heard of it.
HONDA CIVIC HYBRID: Apart from the funny wheels, you may think a regular Civic just whistled by.  Come to think of it; that story rings true with the Civic GX as well, and may contribute to Honda’s lackluster sales in the hypergreen market.
SMART FORTWO: With its very small footprint, the fortwo should be able to leave a… very small footprint, in the carbon sense.
TOYOTA YARIS: Toyota offers smaller options in other parts of the world (Aygo) and will offer a true smart competitor in the near future (iQ).  For now, the Yaris is very economical and just plain good.
NISSAN ALTIMA HYBRID: Nissan can thank Toyota for some of the technology, but very few customers have the option of thanking Nissan for the Altima Hybrid because of its limited geographic availability.
TOYOTA COROLLA: One of the world’s best selling cars of all-time happens to be very cheap to run.  Millions already knew it.
MINI COOPER & CLUBMAN: Kart-like handling and absolute efficiency are not mutually exclusive.
FORD FOCUS: The Focus is aided by fairly impressive handling, but really gains membership by its recognition as a Partial Zero Emissions Vehicle, the only non-hybridized gasoline engined car on the list to go PZEV.
TOYOTA CAMRY: Like most hybrids, the Camry’s efficiency gains may not be enough to help the consumer’s wallet, but that doesn’t change the facts: 34mpg on the highway in a large car.
HONDA CIVIC: Here comes the garden-variety Civic, which along with Hybrid and GX causes one to reconsider the size of Honda’s green-merit badge.
HONDA FIT: Efficiently packaged and inexpensive to buy and run.  What more can you ask for? Membership in The Good 12?