The main bus in the Portland, ME region is the METRO bus.  Many of the buses go to the Monument Square area (Congress St.) to Westbrook, South Portland (including the Maine Mall), and other destinations in the Portland area.  It runs year-round and works best on major roads like Congress St. or Brighton Ave.  Their website is

There is also a seasonal, tourist-oriented bus which takes local visitors almost anywhere they would like to go during their Portland stay, if you wish to go the Mall, the airport, the train station and the Old Port. 

This bus is called the Portland Explorer ( ==).  It has only been running since 2002 but has become efficient during its short life.  It runs daily from May through October beginning at eleven thirty in the morning and running until six thirty at night.  Visitors interested in staying out later (or starting out earlier) will have to supplement their public transportation with taxi service, car rental or walking options but nevertheless, this mode of transportation is found to be useful by many.

This is considered to be an inexpensive option, as it costs only $2 per ride and is free for children under age twelve who must be accompanied by an adult.  It should be noted that this is slightly more than standard bus fare in many cities.  But it should also be noted that, because this mode of public transportation is so tourist-oriented, it is, in many ways, more like a tour bus than a city bus and yet is priced more like a city bus.