Found via Metafilter:
Historic Bridges of the U.S. This is the most complete database of historic bridges I’ve seen. The front page is blog style that seems to have an emphasis on preservation, and which links to a database that is actively being updated & expanded. You can search by state or by county, and look at each bridge’s individual page, including a wealth of stats, and a high-res photo, when available.
Image courtesy of: law keven