OsmAnd

Currently application is the most suitable in these cases :

If you want to visit town (with or without internet) as tourist by foot (you will find all POI and addresses).
If you are osm contributor (you can see osm bugs : open/create, update map, edit POI).
If you are living in country & you often want to search your hamlet, lake, house (street/number), search nearest opened restaurant or closest public transport.
If you want simply navigate to the point specifying lat/lon coordinates & save your track.
If you want to save your favorite points.
If you want use online routing (require a few of the internet data)

For more detailed information refer at the home application page.