I've been map-centric in my research lately. Cartogeeking, really. Here are the sites that have really sucked me in of late:
* Panoramic Maps Collection from the LOC.
* Cultural Landscapes, also from the LOC. Waterway, resource and topographical maps, even some illustrated atlases!
* Rumsey Map Collection
* Perry Castaneda Map Collection
* Historic California Topographical Maps
* Bay Area Historical Topo Maps