When I was in Japan earlier this year, handheld electronics hugely contributed to my ability to navigate the geography and the culture. Or, to be more precise, the software that runs on those devices contributed greatly to the quality of my travels.
My Garmin and me crossing the Sea of Japan
Using my handheld Garmin GPS, I zoomed in on the following map:
The yellow crumb trail shows my arrival at the port, and subsequent walk to Hakata Station (bottom, right hand side).
Rather than paying for inadequate and expensive commercial maps, I used OpenStreetMap's wonderful site to assemble Garmin mapsets. These maps are legally free to all, and are used by commercial services, including MapQuest. They should not be treated as always accurate, but I have had good experiences with them.
Before I left home I also loaded waypoints into my Garmin GPS to help me navigate. I created the waypoints using Google Maps and Bing Maps, so I was able to assemble some accurate points of reference. In some cases (e.g., the TOTO factory in Kokura) I checked the waypoints against satellite and street-level images.
When I arrived in Japan I did not yet have a local data SIM for my smartphone, so Google Maps was not an option. Unlike Google's USA maps, Google's Japan maps cannot be cached ahead-of-time.
An alternative would have been to use the excellent Maps with Me app, driven by my phone or tablet's GPS. I loaded this app, along with maps of Japan into my tablet, and found it useful to review maps on the larger screen of the tablet. However, in the pelting rain of Fukuoka/Hakata, I preferred my tough, waterproof, always displaying, function-key-driven, handheld Garmin GPS.