What are the best options for free historical imagery via services or download? I know NAIP and some other options are available for download, and the latest NAIP is available via service, but is any other nationwide historical imagery freely available for the US?
What you mention is one of the issues with weather, satellite, and many large datasets (i.e., they only show the current data). If you are looking for large scale imagery you can, for example, download LANDSAT satellite imagery (by swath) from http://earthexplorer.usgs.gov and http://landsatlook.usgs.gov . You can also search for USGS DOQ, Aerial Photo Mosaics, etc.
LANDSAT and some other satellites also have "decadal" average data sets (e.g., 1990-99, 2000-2009, etc) that allow you to identify such things as vegetation change over time.
If you are looking for local, small scale, imagery your best bet will be your States Department of Natural Resources or similar.
Try the USGS Earth Explorer.
Specify your area of interest and then under the data sets option, expand the aerial imagery group. I typically click the checkbox next to "Aerial Photo Single Frames" but you might want to explore the other options as well.
In the past, this search returns scanned air photos which are unregistered. All you need to do is georectify them and away you go. I have also found that most of the photos available are at a large scale rather than small scale (but I'm searching up in the mountains through Forest Service holdings).
The USGS has a great collection of about 7Million scanned historical image.
They can be searched for and available from EarthExplorer
In the datasets section select on 'Aerial'. From the site you can download the images and then georeference them in ArcGIS for Desktop. We are also working to make this collection easier to access and directly use.
If you are looking for Landsat data then do review the LandsatService on ArcGIS Online, they contain both new Landsat8 imagery as well as the Landsat GLS datasets going back to 1980. The services are multispectral and full resolution. If you are looking for a more a more comprehensive set of Landsat scenes then check out LandsatLook Viewer . This is based also on ArcGIS servers can one can add the same services to webmaps or ArcGIS for desktop.
An additional source is the United States Department of Agriculture Farm Service Agency - Aerial Photography Field Office (APFO). They produce orders upon request, so you need to have some additional time built in, and they do charge a minimal fee, but still an often overlooked resource.