There are a few occasions when an additive may be beneficial to use. Just so you don't think that all additives are evil, these are a few worthy specimens, although don't expect them to improve gas mileage or performance.
Gas additives can take the form of oxygenates (alcohol or ethers), which can reduce the carbon monoxide expelled into the air by your vehicle. Octane boosters and anti-knocking agents sometimes work, but if you need to use them to have a smooth running engine, then there is a problem with your engine that needs a mechanics attention.
Fuel stabilizers and antioxidants can be helpful if you do not use a vehicle very often, as these will help the gas from getting old and stale and subsequently performing sluggishly. Fuel system cleaners can also be helpful on occasion to clean-up harmful build-up and keep your engine running as it should.
Fuel cleaners will not give your vehicle anymore power or fuel efficiency that they were meant to have, so don't let their tricky advertising fool you. Gasoline additives can also be used to inhibit corrosion and lubricate the upper cylinders, although in general these should not be necessary under most circumstances.
Fuel efficiency cannot be bought in a bottle, it is something that must have come with your vehicle in the first place. Don't use these additives as band aids to fix real problems that need either a mechanic, or perhaps a different vehicle all together.
[i]Feel the need for sustainable speed? See what eco-engineers are doing to create super-charged but low-impact cars, bikes, and planes on Planet Green TV's Mean Green Machines.