Some Android apps for viewing offline maps
Only free apps are listed. Will review later.
- TrekBuddy (classic app, available on many platforms, its map format can be used by all the above apps). Its official website.
Download offline maps
There are a few software and web services for downloading maps from services like Google Maps, Microsoft Maps, etc, to computer files. However, probably the best and most popular one is Mobile Atlas Creator (formerly TrekBuddy Atlas Creator). For TrekBuddy and RMaps, just download the TrekBuddy-format maps and put them in corresponding folders on the SD card. For Locus, here are the instructions.
To download navigation directions from Google Maps, a very useful web service is GMapToGPX.