Technology based solutions generally modify some aspect of the internal combustion engine (ICE) to either reduce emissions or enhance engine efficiency. Solutions available for heavy duty vehicles are generally not available for light duty vehicles, though some light duty solutions, such as hybrids and electric propulsion, are becoming more common on heavy duty vehicles.

Light Duty Vehicles (one to four wheels)
for Consumers

EFFICIENT VEHICLES: off-the-shelf energy saving technologies such as the use of lighter weight materials, tire pressure monitoring systems, real time fuel economy feedback, vehicle right-sizing, & cylinder de-activation.

ELECTRIC: designed from the ground up to operate on electric-only propulsion which is powered by a battery. generally limited by load capacity or distance between charges due to limitations in existing battery technology. current models include Myers Motor's NmG made in Tallmadge, Tesla Motor's Roadster, and the Tango Commuter Car.

HUMAN POWERED MACHINES: bicycles, tricycles, and velomobiles are  to this day, the most efficient means of human powered transportation!

HYBRID ELECTRIC: small electric motor coupled with battery and internal combustion engine. this system enables internal combustion engine (ICE) horsepower to be supplemented by an electric motor while enabling the battery to be recharged by excess engine power as well as heat energy from braking friction. hybrids can typically run in a variety of modes including electric only, electric assist, and engine shutdown, the latter which enables the ICE to completely shutdown at stoplights. A variety of models are currently available with many more on the way. You can compare current models at FuelEconomy.gov

NEIGHBORHOOD ELECTRIC: neighborhood electric vehicles (NEV) are a class of vehicles restricted to 25mph. initially designed as glorified golf carts, they are becoming more car like as they gain popularity as a second car for urban dwellers.
Manufacturer's include GEM, ZENN, & Columbia

PLUG-IN HYBRID ELECTRIC: the best of a merger between hybrids and electric vehicle technologies. enabled by a larger battery that can be plugged into the grid at night, thereby resulting in a vehicle that can be driven much farther than current hybrids in electric-only mode.
No models in production yet.

Heavy Duty Vehicles (four to eighteen wheels)
for Business and Industry
Link to Diesel Retrofit Technologies verified by the U.S. EPA

AUXILLARY POWER UNITS: a small motor or engine that allows a vehicle to maintain in-cab heat and other amentities without idling the engine.

DIESEL CATALYSTS (DOCs): reduction of various emissions by chemical neutralization, primarily through the use of precious metals. catalysts are limited by engine operating temperature.

DIESEL PARTICULATE FILTERS (DPFs): reduction of various emissions by entrapment, usually via a ceramic filter. entrapped emissions must be periodically cleaned-out and disposed of.

TRUCK STOP ELECTRICIFICATION (TSE): truck stops offering electric-power so that truck drivers can access amenities (air-con, internet, heat, etc) withot needing to idle.

FUEL CELLS: replaces ICE with electric motor powered by refillable 'battery' fueled by hydrogen.