The first American flag was raised on Prospect Hill in Somerville.
Fluff was also invented there by a guy named Archibald Query, although I understand that he ultimately sold off all rights to his invention for a thousand bucks. Poor guy didn't know what he had.
Whitey Bulger, the real-life fugitive who inspired the Jack Nicholson character in The Departed, was raised in the Winter Hill section of Somerville.
The deal with Somerville is that the nicer parts are near Davis Square and the Red Line, while I lived on the shadier part (like most of Somerville - shady) near the Orange Line. However, the Orange line side is super convenient for downtown commuters. My subway ride to downtown was 8 minutes from Sullivan Square, while you're lucky to get downtown from Davis Square in 25.
The next hot area, theoretically, is Union Square, which is right in the middle of the city. The Green Line is supposed to be extended there by 2013... of course, that's in "Boston" years, which means they'll be lucky to have it there by the 22nd century. Once the green line is certain to get there, however, property values will skyrocket.