Good stuff but it can't top the Delaware Geological Survey's georeferenced
maps from the 1868 state atlas:

"Delaware's hundreds as TIFF images zipped together with their associated
world file and metadata. Some files are as large as 23 megabytes, but most
are in the 1-2 megabyte range."
<http://www.udel.edu/dgs/dgsdata/hundreds.html>.

I've been using them in the field in combination with my GPS unit and I
can't think of a more useful FREE resource.