This app was mentioned in 3 comments, with an average of 9.33 upvotes
For Paris it's easy. There are something like 500 metro stations, you can get anywhere with public transportation and even if you don't know the city you won't get lost, just download the app
The official RATP app has a downloadable offline map, great if you don't want to burn up roaming data. It still uses GPS so great for going off piste.
https://play.google.com/store/apps/details?id=net.ixxi.ratp.tourisme
Whe I was in Paris last year we all got evacuated from the Metro because of smoke in the tunnel. Without this app I would have been stuck. Magnifique!
Here's my tip for people visiting Paris: download the Paris Metro app. It works off-line, no need to have a data connection. That, plus a Paris Visite pass is all you need to go anywhere in a city that has an awesome public transport system.