Another caveat- I use these apps on Apple devices, so I know that they are available through the Apple App store. They may well also be available for Android devices, and all the ones I am listing are free (or free versions).
Currency exchange rates (note that these rates don't include service charges we'd have to pay): XE and their website is www.xe.com
Translation app (look up the words/phrases you anticipate needing while connected, it stores them for you): Google Translate
Translation of popular words: French (from lastminute.com)- it works off-line! (The app's icon looks like the French flag: blue/white/red vertical stripes)
Free French Lessons: Duolingo (thanks to a parishioner for telling me about this!)
Information about France: Rick Steves Audio Europe - then add selected interviews to your playlist for later listening
Portable guidebook: Ulmon Paris - personally, I find this a little confusing to use. Access the magnifying glass icon to get to what I'd term the 'home page'. It will show places on a map, and show you a Metro map, too.
Okay- what are YOUR favorite travel-related apps?