As much as we liked the Chevy Volt, it's only really efficient when it's running on electricity. When you kick the gas engine in as a middleman, it gets much less eco-friendly, but a new type of fuel cell is able to convert gas into electricity. And it's tiny.
Traditional fuel cells use fuels like hydrogen or alcohol to generating electricity. A solid-oxide fuel cell (or SOFC) also generates electricity from fuel, but you can put just about anything you want in there, from gasoline to diesel fuel to natural gas. SOFCs have a power-to-size ratio that's 10 times better than other fuel cells, and if you put gasoline into them, they're just as efficient at producing electricity as a gasoline-powered generator in a tiny fraction of the space.
The only reason we're not all using these things right now is that they have to be heated up to 1,600 degrees to get them to work, but new designs have dropped the operating temperature down to a much more reasonable 600 degrees. A SOFC stack about four inches on a side would be enough to power a car (!), and if you combined it with a battery system, you'd get a plug-in hybrid like the Volt except more efficient and with an almost entirely empty engine compartment.
At present, the big disadvantage of SOFCs is that they don't work until you heat them up, which means cars that relied on them wouldn't be able to turn on and drive instantly. But again, the idea is that they'd be combined with a battery that would step in while the fuel cell heated up, and that the SOFC would then start recharging the battery, just like the Volt's engine does.
The first prototypes of small, relatively low-temp SOFCs for commercial use are probably going to end up powering the cabs of long-haul truckers, where they'll provide electricity for air conditioners, microwaves, and PS3s 85% more fuel efficiently than idling the truck's engine. The first tests should start next year, and once the system has proved itself, getting them into our cars will hopefully happen soon after.