Human powered trikes and quads are interesting vehicles. Some people strongly prefer them for comfort and they seem to get more attention from drivers on the road and can be a bit more aerodynamic. On the other hand they tend to be heavy and can be "interesting" to pedal up a hill (you can't come out of the seat and stand on the pedals) - but for some people they may be ideal.
A friend in California is tinkering with a quad with an aerodynamic shell on it to make a somewhat lower cost velomobile. Velomobibles, with their more aerodynamic design, can be extremely efficient and can often be ridden about 10mph faster than a normal bicycle. They are very rare in the US and present some real challenges, but probably represent a great project for the dedicated amateur. If gasoline became seriously expensive, velomobiles would be a very interesting area to explore for local (up to 10 miles) transportation and costs could be dramatically decreased with volume production. The Dutch are considering how to accommodate them in their long range plans - much of the design activity is in the Netherlands and Germany.