Maps.Me (formerly known as Maps With Me) is a straightforward app which displays detailed offline maps of all countries in the world.

Zoom in on a country, tap the "Download" button, and Maps.Me grabs the necessary OpenStreetMap data for you. Maps then work everywhere, no need for an internet connection, no roaming charges, no lengthy delays while you zoom in: it's all very quick and easy.
The maps are extremely detailed, too, covering restaurants, railway stations, tourist spots and a whole lot more.

The Lite edition is strictly basics-only. You're able to zoom in and out, your current location is displayed and updated via GPS, you can share it with others (if you are online, anyway), and that's about it.

Still, this is more than enough to be useful, and the $4.99 Pro edition adds some essential extras: offline map searches, location bookmarking (including import/ export to KML/ KMZ), and an auto-follow mode where the map automatically orients itself to the direction you're travelling. Tap "Upgrade" on the app menu to find out more.

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The free version may be short on features, but Maps.Me is also fast, easy-to-use and works everywhere. The maps are so detailed that it's worth installing as a backup, even if you prefer some other mapping app.