Historic Pittsburgh is a site featuring documents, maps and books from the University of Pittsburgh and other Pittsburgh-area collections. Their Map Collections section has four large series of map scans available:

• Geodetic and topographic survey maps for Pittsburgh between 1923 and 1961;
• Maps from the 1912 Flood Commission for Pittsburgh;
• Plat maps for the greater Pittsburgh area from 1872 to 1939 in the G. M. Hopkins collection; and
• The Warrantee Atlas of Allegheny County from 1914.

The total is over 1,300 plates (mostly from the Hopkins collection). Thanks to peacay for the links.