This is Watford Museum, housed in the old Benskin's office building at the bottom of the High Street, next to Watford High Street Station. It has been open for about 10 years, and gives a very good historical view of the town. It contains many exhibits and old photos, and admission is free.
It is located at position 16 in green on the map.
Tel: 01923 232297 / 226803