JBG, USGS ( U.S. Geological Survey) maps are indeed available free, now that we no longer need to get them printed on dead trees. Caltopo.com is a good website for viewing them. I believe the British equivalent, Ordnance Survey maps, are more closely guarded, but I'm sure there's some kind of free alternative, or the data has leaked out onto the Internet somehow.
Back when I made more use of my GPS receiver, I found free maps (one file per state) for that too. The program I us most now on Android is Locus, and there are free maps for that too. It's also able to use topo maps, but I haven't found a source for them that gives coverage for a wide area all in one download. However, one can certainly get enough of a map for a region covered by a few days' walk.
The GPS receiver has a tiny screen, but it's easily readable in daylight. My phone has a bigger display, but it's tough to see it in sunlight. The range of maps available for it is irresistible, though. I think of the phone as "a GPS unit that you can make phone calls on".