I searched an hadn't seen a post on this new service yet so I figured I would share. A colleague of mine(landscape architect of all things) mentioned this to me and figured I would put it out there for everyone.
The USGS has released their TopoView service, where you can easily search for historic USGS Topo Maps from 1880 to present.
This is by far one of their slickest and easiest to use services, you can search maps from custom time ranges, and the scales you need. Another huge benefit is that all of the topo maps can be downloaded in multiple formats, JPEG, Geo-PDF, Geo-TIFF, KMZ, as opposed to only being able to get Geo-PDF that was available through the USGS National Map Service. This was mentioned in Wired magazine website(apparently causing some server issues due to a flood of people checking it out)(original article here It Just Got Easier to See a Cool Historical Maps Collection | WIRED ) . This is a great example of being able to make data available to the public for users, both those GIS users and non-GIS users.
Wired's article also mentioned the USGS was hoping to make the USGS geologic maps available through a similar service or an the existing service, which I for one am very interested in seeing.