The answer starts with defining what "realistic alternative to gasoline" means. If your definition is like mine, it means "which fuels have the most realistic chances of significantly reducing the use of foreign oil".
Put simply, we don't have nearly enough capacity to produce biofuels at a level that would make a serious dent in the global use of oil. Biodiesel requires the growth of crops, then processing of these crops into fuel. We simply don't have enough farmers to produce the crops and plants to process into fuel. The lead time to improve capacity is far too long.
A similar issue plagues E85 ethanol. While there are more corn growers, a global switch to E85 would require every piece of American farmland to grow corn for ethanol within a decade to support the fleet. Ethanol is not a great fuel, because it is less efficient and produces less hp. People point out that it is cheaper at the pumps, but this is entirely because of large current supply and low immediate demand.
Many "green" types love traditional diesel, because of the lower greenhouse gasses. Unfortunately, diesel helps the long-term health of the ozone layer for an immediate hurt of people's lungs. Diesel is nasty to human lungs due to the particulates produced during combustion. Diesel particulates are a major source of asthma problems around the world. Luckily, cleaner diesel fuel ("low sulfur") and better technology from automakers like Mercedes, VW and Audi have helped significantly, but these engines require frequent maintenance to keep the urea solution (the particulate filter) clean.
Another downside of diesel is that it is still oil. As mentioned previously, biodiesel cannot be made in enough supply to support the low numbers of American commuting consumers, much less the global commercial market. Additionally, diesel prices can vary wildly -- oftentimes costing far more than gasoline.
The idea of electric cars is older than combustion engine cars. Electric cars in 1900 had the same problem as modern electric cars: range and recharge time. Battery technology is still as far from allowing the range consumers need now as it was then. (200 miles in a Tesla is actually less usable today with urban sprawl than an early 1900's Detroit Electric with a 30 mile range for a person living no more than a mile or two outside of the city.)
Hybrids are a good compromise, but again, this a technology that is nearly as old as cars themselves. The first gasoline-electric hybrid was created by Ferdinand Porsche for the Lohner company in 1903. By around 1904 Porsche added regenerative braking to the vehicle. Other fuel-electric hybrids existed before 1910.
Toyota made hybrids "cool" with the Prius, but it is just a small step. From a green perspective, there's more fuel and pollution associated with the production and transportation of the batteries for installation in Prius vehicles than a domestically made (either domestic or foreign-owned) vehicle uses and makes in a typical usable life.
Hydrogen fuel cells have been a pipe dream for fifty years. Read technical and automotive magazines from the 1950s, 1960s, 1970s and you'll find articles that all say the same thing "we're still a decade away from making hydrogen cars a reality".
What really has a shot at being a good alternative are plug-in hybrids. Bob Lutz, former Vice President of GM, told me last year that it is GM's intention to make the plug-in electric/gas hybrid technology in the upcoming Chevy Volt the standard powertrain in all of the company's front-wheel-drive vehicles.
The upsides of plug-in hybrids are big: forty miles of electric power gets most drivers to and from work/school without touching the gas engine, but unlike electric-only cars, the gas engine gives another 300-plus miles. This significantly increases the average mpg.
If we take it a step further, figure battery technology in the next five years gets the car to 75 miles, and the use of diesel engines increases the mpg, we could see a functional average of 200mpg returned by a standard mid-size sedan in typical urban usage.
Of course, there are downsides! The electric grid is not capable of handling a million or two plug-in cars added each year. Consequently, it will take investments in new powerplants and other infrastructure. (Anyone want a coal or nuclear energy plant in their town?) Furthermore, don't expect the cost of driving on that electrical energy to be cheaper than $3-per-gallon gas.
In the end, there are no easy answers. Part our struggle is that automakers had no reason to innovate over the last century, so we're not much further down the road to a solution than automotive engineers were a century ago when electric and steam cars outsold gasoline-powered vehicles!