Abraham Schechter at the Portland Public Library has done a nice work in getting a couple of often-used Portland atlases into a digital format. First, there's the Goodwin Atlas of 1882. Mr. Schechter writes, "Having been produced shortly before the merging of the municipality of Deering with Portland (in 1899), the Atlas provides a detailed documentation of the Portland peninsula (comprising the city), between the aftermath of the Great Fire (1866) and 1900, when the city boundaries extended, and a number of street names had to be changed."
In addition to the Goodwin Atlas, Mr. Schechter has also digitzed the 1914 Richards Atlas of Portland. This useful atlas, which is also held in the OML collection, contains "24 double page maps in color, documenting the entire city, showing each property's lot line, building, many owners' names, electric and steam railways, sewer lines, water mains, etc. Areas of each lot in square feet and acres."
Again, an interesting post of the PPL blog depicts the whole restoration and digitization process.