One of the better Civil War museums around is the Fifth Maine Regimental Museum on Peaks Island (in Portland harbor), Maine.
The building was originally called the Fifth Maine Regiment Memorial Hall when it was built in 1888 as the "Headquarters" of the Fifth Regiment Maine Volunteer Infantry 1861-1864. For nearly sixty years the veterans and their families summered at the headquarters, enjoying the cooling ocean breezes and magnificent view from the verandah of their beloved cottage.
As the veterans aged and eventually all passed away, ownership of the building passed to the Fifth Maine Regiment Community Association. The Hall is now maintained as a Civil War and local history museum and a cultural center for the island community. In 1978 it was listed in the National Register of Historic Places and, in 1984, was designated as a Greater Portland Landmark.
The Fifth Regiment Maine Volunteer Infantry was one of the first Maine regiments to be mustered in. The regiment consisted of 1046 men from southern and central Maine (Another 500 joined later.). Three of the ten companies in the regiment were composed of men from Portland, thus earning the regiment the nickname the Forest City Regiment.
The Fifth fought in 22 battles from First Bull Run (Manassas) to Petersburg. During the battle of Rappahannock Station, the regiment is credited with capturing 4 Confederate battle flags and 1200 prisoners (including several officers). The Fifth was known as one of Maine's fighting regiments. It captured more prisoners than the number of men who served in the regiment and three times as many battle flags as any other Maine regiment. After three long years, only 193 men were mustered out in July 1864. The 1,500 or so had been killed in action, died from disease, wounded, deserted, or transferred to other regiments.
The museum maintains an active web site. You can read more about this active museum at http://www.fifthmainemuseum.org