Not sure if you guys know about this or not but this resource may be of interest to you:
USGS has recently digitized and made publically accessible their entire historic topographic map collection for the US for free. There's a few states or parts of states that aren't finished yet but will be soon. You can view and download the full size maps to your own computer and edit, print, etc. from there. USGS also provides their own map viewing/editing software for free (GEO-PDF).
In some cases (including parts of Glacier National Park), there are historic maps dating back to the early 1900s. It's a pretty amazing way to see how the trail networks, camps, ranger stations, etc. have evolved over the decades. In the case of my local backcountry, it's been eye opening to see how many trails have come and gone and to find the locations of historic cabins, campsites, etc., some of which are still out there in varying degrees of decay.
The next step, which USGS is working on (some states are already done), is to make the new 2011 7.5 quads available for free download as part of the same collection.
Here's the home page for the USGS map archives