Boston Common consists of a large and open, but irradiated area. The common contains a few other locations, such as the Massachusetts State House to the north, Swan's Pond to the western side of the green, the Boylston Club - the red door in the southern row of buildings off of the common, the Prost bar - which is a basement bar on the north corner of the western row of buildings, and Park Street station which is located at the north-easternmost corner of the common.
The Freedom Trail also begins at Boston Common. There is a friendly protectron named Tour bot located on the eastern side of the common next to the fountain. In front of the fountain there is a sign and a seal. The sign reads: "At Journey's End Follow Freedom's Lantern". The seal says "The Freedom Trail. Boston." But there appears to be a code. The number 7 is written in the center with red ink, and the letter A in Trail is bracketed. Each of the seals has a number that corresponds to a letter in "Railroad" - for example the seal marked 1 has the "R" indicated.
Grognak the Barbarian issue #6 - On the ground in the gazebo at the center of the common next to a skeleton and (severe) radiation barrels.
The Boston Common only appears in Fallout 4.