Hybrids are alright if you do a lot of stop-and-go city driving. All my greenie friends have told me for years that bio-diesel is the way to go -- particularly if you like a car that smells like McDonald's french fries.
But since I drive a '92 Nissan that's rated for 39mpg/29mpg city and only has 84k miles on it, the greenest thing I can do is let that puppy rust in the street.