A friend loaned me several copies of the magazine "Overland Journal", in particular Summer and Fall of 2010. This is a truly excellent magazine, full of unique information and very nicely produced. These issues covered mapping software for Windows and Mac systems, anticipating that users would be carrying laptops into the field (though these days, tablets are much more likely and far more appropriate). Oddly enough, these articles do not mention NGS TOPO! The following list can server as starting points, if desired, for further searches.
The articles point out that there are two strategies with mapping software. One is to provide both software and maps (and in most cases lock you in to them as your source of maps). The other strategy is to provide software that allows you to ingest maps that you obtain online or scan yourself. An interesting distinction and strategy that I was not aware of.
Gtopo / firstname.lastname@example.org