2009 BMW 335d
Yep, that's right, the Internal Revenue Service will actually give you up to $900 to buy a 2009 BMW 335d. Even better, up to $1,800 for a 2009 BMW X5 Xdrive35d Sport Activity Vehicle (as BMW pompously insists on calling it).
It's all courtesy of the Advanced Lean Burn Technology Motor Tax Credit, which subsidizes vehicles that burn a lot less fuel and emit much lower levels of greenhouse gases. It comes to you from the Energy Act of 2005, with various vehicles qualifying each model year--which the IRS helpfully lists on its website (though as of today, hey hadn't updated the page to include the ultimate driving machines).
The news comes courtesy of BMW, who launched two diesels into the US market just in time to see diesel soar to roughly $1 a gallon more than gasoline. Diesels are pricier to build than gas engines--they have to be sturdier to withstand the much higher compression ratios--but usually that's offset by better fuel economy. Now, the price differential makes the math a real challenge.
Neither diesel is exactly a slouch, by the way. The 335d sedan does its 0-60 mph in just 6.0 seconds, while the porkier X5 diesel (I'm not repeating the gibberish model name, sorry) is just slightly slower at 6.9 seconds. The 335d is the most fuel efficient sedan BMW has ever sold in the States, racking up EPA ratings of 23 miles per gallon in the city and a blissful 36 mpg on the highway (the X5 comes in at 19/26). Either one will probably outlast your bladder, as both can travel almost 600 miles on a single tank.