Motorcycles are also great, but harder to learn to ride, and less maneuverable in congested urban areas. (Motorcycles typically are standard, so you have to shift gears, whereas most scooters have continuous variable transmission i.e. automatic)
Most new scooters have very high mileage -- 100 MPG is not uncommon, and some are even better. I found a link here to a source of more detailed data: http://www.greenconsumerguide.com/scoot.php
In some cities (like mine, Toronto), scooters park free on city street parking. Even the cheapest vehicle parking costs me about $15 just to get in and out of one client meeting; if I park underground, it quickly rises to $25. There are lots of convenient places to park the scoot. Last summer I estimated that I saved about $300 on parking, which is a pretty good rate of return on the investment, considering I only ride it 6 months of the year.
Newer scooters have low emissions. Those built to European standards are very low, as they only permit 4-stroke engines. A newspaper here reported this startling statistic:
If Americans used scooters for just 35 per cent of their weekly driving, they could, in aggregate, reduce fuel consumption by 53 million litres a day, ICR found, citing U.S. Department of Energy data. They could also reduce carbon dioxide emissions by 147.2 million kg a day.
Plus it's fun to scoot, you meet a lot of nice people, and if you want, you can join a scooter group and attend rallies and go on group rides.
This is really barely an instructable, but I wanted to add this perspective to the green contest.