http://geography.about.com/od/lists/a/placename50.htm
the place name of Riverside can be found in all but four states (it does not exist in Hawaii, Alaska, Louisiana, and Oklahoma).
The runner up was Centerville in 45 states, followed by Fairview (43 states), Franklin (42), Midway (40), Fairfield (39), Pleasant Valley (39), Troy (39), Liberty (38), and Union (38). Springfield isn’t even in the top ten (only 35 states have a Springfield).