Hartford County, Connecticut Online Historical Directories – Others, as well as the 1899 directory above.
CT State Library’s Flickr photostream – Others of interest, but especially the Thompson photo set.
William H. Thompson Photographs of Hartford, 1903-1906 – a set from the CT State Library (above). 99 black and white photographs of central Hartford. many from the now-destroyed Front Street neighborhood, bounded by Morgan Street, the river, State Street, and Main Street. Mostly taken at intersections, of the streets themselves, so the buildings are not central to most views.
Connecticut Historical Maps and Atlases – Very detailed (building by building) maps of Hartford and other cities. Hartford maps from 1917, 1909, and earlier.
Hartford Public Library Digital Collections – Navigation at the Hartford Public Library site is very tricky. But this link (as of Jan. 2013) links to two nice photo sets: Hartford: The Making of a City, 1900-1930, and Reflection of The Times (photos from The Hartford Times between 1950-1976).