Histografica

Histografica is an interesting resource of user contributed media - they are aming to build a large collection of historical photographstied to the geographic location and time period; so you can search for photos by both time range and location. 

For exmple, searching on Washngton DC pre 1904, I found this photo of the Smithsonian Institution from 1898

You can help grow it by either sharing your photos or adding helpful information to exisiting ones via the comments form.

Histografica http://www.histografica.com/