Not an app, but the best thing I did was email myself walking directions to places I wanted to go from google maps. Always accurate and we never took the metro and took only a couple of cabs. Great way to explore the city and you discover so much along the way.

On my walk to Notre Dame I planned that we would stop at Berthillon for the most amazing ice cream.

Louvre, the love lock bridge (pont des artes) and then cross over to St. Germain on another walk

Michelin has apps.

It was a few years ago so take it with a grain of salt for both memory and outdated reasons) but a couple stand out-- escargo for menu translations, metrO for navigating public transportation (works in all major cities actually), I think we used the Rick Steves tour for Versailles, the Louvre has a nice app, (and one more whose name is escaping me, sorry).

Anything and everything. Thanks!