Organic Maps is a free Android & iOS offline maps app for travelers, tourists, hikers, drivers and cyclists based on OpenStreetMap data created by the community. It is a privacy-focused, open-source fork of Maps.me app (previously known as MapsWithMe), maintained by the same people who created MapsWithMe in 2011.
Organic Maps is one of the few applications nowadays that supports 100% of features without an active Internet connection. Install Organic Maps, download maps, throw away your SIM card, and go for a weeklong trip on a single battery charge without any byte sent to the network.
It looks like there is also a desktop version.