A technology which allows internal combustion engines to run on cheap American beer... err...
The Miller Cycle, developed by American engineer Ralph Miller in the 1940's, is a modified Otto Cycle that improves fuel efficiency by 10%-20%. It relies on a supercharger/turbocharger, and takes advantage of the superchargers greater efficiency at low compression levels. As with other forced induction engines more power can be had from a smaller engine, but without the efficiency penalties usually associated with forced induction (e.g. a Miller Cycle v6 can get the power of a v8 yet still retain the fuel efficiency of a v6).
During the intake stroke the supercharger overcharges the cylinder with fuel and air, and during the first bit of the compression stroke the intake valves are left open and some of the overcharge is pushed out. While the overcharge is being forced out and until the intake valves close the piston isn't pushing against anything and in effect the compression stroke is shortened compared to the 'normal' power stroke. While the supercharger normally employed does use some of the engines power, it's much less than the power saved from the shortened compression stroke. The lower friction associated with the smaller engine also improves efficiency.
While the Miller Cycle is widely employed in large-scale applications (such as generators and ship motors), the only manufacturer to ever sell a Miller Cycle powered automobile is Mazda with their Mx-6 and Millenia/929 (both of which are out of production in North America, although the Millenia is still being sold in Japan).
I don't know why Miller-powered vehicles haven't caught on (anyone want to explain?). While Mazda's weak marketing surely shoulders some blame (despite high praise the Millenia was a perennial under-seller), that still doesn't explain why other manufacturers haven't also offered Millers for sale.
Perhaps consumers are simply unwilling to pay more up front for better gas mileage - even if it saves far more in the long run. Superchargers cost money, and somewhat higher power (at the cost of lower fuel economy) can be had running a normal supercharged Otto cycle. If this is the case, variable valve timing technology (notable continuously-VVT) offers the prospect of allowing engines to run in both thrifty Miller-mode but also revert to a normal supercharged-Otto-mode when more power is needed. | 
