Extengine Transport Systems wants to be the world's smog fighter. The company, based in Fullerton, California, retrofits large diesel-powered vehicles such as garbage trucks and off-road construction equipment with a system that cuts smog-causing emissions by as much as 90 percent. The system injects liquid urea into the vehicle's exhaust stream, where it breaks down nitrogen oxide into harmless nitrogen and water without diminishing engine performance or fuel efficiency.

Founder and CEO Phil Roberts spent the early part of his career at Arco Petroleum Products, converting the company's standalone gas stations in California into highly profitable convenience stores. But selling gas, Twinkies, and cigarettes didn't sit well with his conscience. He quit to explore different ways to reduce emissions, finally zeroing in on diesel vehicles. "I guess I got stuck behind too many school buses and trucks," he says. "I was amazed that these vehicles were even allowed on the road." They may not be for long. A mandate in California requires that commercial diesel vehicles reduce emissions significantly by 2020. Customers such as Waste Management and the city of Houston have already installed Extengine's system on some of their vehicles. A new system under review by the EPA and the California Air Resources Board will plug and play on virtually any heavy-duty diesel, including public buses. Roberts, for his part, commutes in a diesel pickup truck retrofitted with his system.

"Electric cars were made for people who thought that driving was a necessary evil," says Martin Eberhard, co-founder and CEO of Tesla Motors. With the Tesla Roadster, unveiled this past summer, Eberhard, 46, has made a car that is necessary but decidedly not evil. The stylish two-seater hits 60 miles per hour in four seconds, as fast as a Ferrari--but at $100,000, it's considerably less expensive than one. It's also a blast to drive and can go up to 250 miles on a three and a half hour charge.

The idea came to Eberhard after he and co-founder Marc Tarpenning sold their e-book manufacturer, NuvoMedia, to Gemstar for $187 million in 2000. "I had enough money to buy a fancy sports car, but I wanted one that was ecologically responsible," Eberhard says. He found an eager partner in PayPal co-founder Elon Musk, who has invested some $30 million in the San Carlos, California, company.

Rather than hiring designers and building a plant like a typical small car company, Eberhard outsourced the manufacturing. The electric motors are built in Taiwan. For the power technology, he licensed a prototype and focused on developing software to keep the 6,831 lithium ion batteries cool at high speeds (thus avoiding the fate of an overheated Dell laptop). Advertising costs so far have been close to zero: Eberhard and Musk simply set up a blog. Finally, instead of negotiating distribution agreements with dealerships, Tesla will sell directly to consumers through its website, possibly using an eBay-inspired auction format. It's an ambitious and untested plan, but the results are already encouraging. Tesla sold its first 100 Roadsters sight unseen in just three weeks, and in July raised an additional $40 million from investors including Silicon Valley VCs VantagePoint Venture Partners and Draper Fisher Jurvetson, as well as JP Morgan. Other investors include Google founders Sergey Brin and Larry Page and former eBay president Jeff Skoll. By 2009, the company plans to offer a $50,000 luxury sedan. Eventually, Eberhard foresees an electric car priced at about $30,000. "I'd like us to be a major American car company," he says.

U.S. truckers are required to rest 10 hours for every 11 they drive. But if they turn off their engines for that stretch, they'll bake in the heat or freeze in the chill of their cabs. A motel room would only shrink their earnings and keep them away from their load. Unfortunately, idling trucks waste fuel, spew diesel fumes, and suck in carbon monoxide. They also get ticketed in most states.

A.C. Wilson, a scrappy inventor in Knoxville, Tennessee, thinks he has a solution: Wire every truck-stop parking space with heat and AC and lure drivers to plug in with phone service, high-speed Internet access, satellite TV, and movies on demand. The IdleAire system consists of big yellow accordionlike tubes that plug into a dock installed in the cab window. Drivers pay for the services with smart cards. The current rate: as low as $1.85 an hour. The company claims its service conserves one gallon of fuel per idling hour per truck, and with more than one million truckers on the road at any given time, the savings add up--so far, says Wilson, eight million gallons of fuel have been saved, 83,000 metric tons of emissions have been eliminated, and 150,000 road cowboys have plugged in.

When it comes to greenhouse gases, carbon dioxide emissions get all the attention. But methane gas, which is produced by, among other things, rotting garbage in the nation's landfills, is 22 times more potent a contributor to global warming. Most large landfills simply burn their methane--which not only pollutes the air but also wastes a potential source of energy.

At least that's the way Prometheus Energy sees it. The Seattle-based company has developed a process to capture methane gas, purify it, and convert it into liquefied natural gas, or LNG, which can be used to power buses. Prometheus' first commercial project, currently under way at the Frank R. Bowerman Landfill in Orange County, California, is designed to produce 5,000 gallons of LNG a day and save emissions equivalent to removing 15,895 cars from the road per year, according to the U.S. Environmental Protection Agency.

Founded in 2003 by CEO Kirt Montague, a lawyer, and Cary Wasden, a banker, Prometheus is trying to do more than simply create a new source of energy. Prometheus is striving to create a new model of "distributed fuel," in which fuel is produced close to where it's used. Buses in Orange County, for example, already run on liquefied natural gas, so the gas from the landfill could be used to power them, saving more energy and eliminating more pollution. Meanwhile, in addition to the mountains of trash to be tapped, there are plenty of other sources of methane--stranded gas wells, coalbed methane, and even the biogas emitted by cow manure--that Prometheus plans to capture and put to use.

Not many automotive companies crow about how many vehicles they've managed to retire. But that's one of the ways Zipcar, the country's largest car-sharing service, measures success. Based in Cambridge, Massachusetts, the car rental service has a fleet of 2,000 vehicles, 10 percent of them hybrids, available to drivers from Toronto to Minneapolis to Boston and Washington, D.C. Some 40 percent of Zipcar's 70,000 members say that by participating they've avoided buying a new vehicle or gotten rid of an old one.

Since it was founded in 1999, the company estimates it has taken 25,000 cars off the road. Members pay a $25 application fee to join and as little as $7.50 an hour, or $51 a day, for a car, picking up the vehicles from parking spaces in their neighborhoods without ever interacting with a clerk. According to company surveys, the average Zipcar member drove 5,295 miles per year before joining the service and now drives just 369 miles annually. "If you bought 500 pounds of candy you would be more likely to eat more of it than if you had to buy one pound, 500 times," says president and CEO Scott Griffith. "The same sort of effect happens with car sharing."

And car sharers are lining up to drive less. For the past two years, membership has grown 100 percent annually, and revenue, $15 million in 2005, is expected to double this year.