In a follow up post to Ross' pervious hybrid vehicle post, there are some interesting hybrid vehicles coming down the pipe from various car manufacturers.
Ford is rumored to be working on a revolutionary "hydraulic hybrid" which would make a huge difference in the current mileage of hybrid cars at 3x the current battery powered fuel efficiency. Wow!
If you want to do some comparisons on hybrid vehicles (or just look at the fuel economy of current makes of cars you might be thinking of) the US Government has put together a nice resource at http://www.fueleconomy.gov. If you want to directly compare the current crop of hybrid vehicles, you can do that too.