Please be sure to check with your phone provider regarding any charges that you may incur for data and/or phone calls while abroad. Some companies will have special plans with data, text and/or voice for short-term travel. There is free wifi in the dorms, as well as in some public parks and stores.
CityMapper: CityMapper helps you navigate town by providing public transit information, maps and even expected costs for taxi rides. You can put in your destination and CityMapper will tell you the different ways of getting there. And, if you are on the Metro, it will buzz your phone to let you know when to change trains!
RATP: This is the official app for the Paris transit system. Like CityMapper, you can type in your destination and see which options (bus, metro, tram) will get you where you want to go.
Uber: Works just like in the U.S. with charges going directly to your credit card. Note that your driver may not speak English. Be sure to type in your destination address precisely!
XE Currency Converter: Not sure how much that item costs in U.S. Dollars? This app will let you figure out what the approximate cost is in $$. Note that your cost may be higher because (a) you are not getting the bank-to-bank currency exchange rate in most cases and (b) your bank or credit card may charge a fee for currency exchange.
Google Translate (iPhone, Android): You can type something in or even use your phone's camera to get a translation. It's not 100% accurate (don't try using it for your language class homework!) but it will give you what you need to know in most circumstances. Note that this only works when you have wifi access.
Metric Converter (iPhone, Android): Can't figure out if 24 degrees means it will be hot or cold? Don't know how many grams are in a pound? Metric Converter has an answer for those questions.
Flush Toilet app in English for iPhone or Android.) : When you gotta go, you gotta go. It uses a map to help you find the closest potty.
Paris Map Offline (Android): Unlike the app above, this one won't use up your battery quite so quickly. It has some information about the different monuments. If you are online and use GPS, you can pinpoint your location (but it's slow).
Rick Steves Travel Europe: Gives you audiotours, supplemented by maps, for cities around Europe. No wifi connection needed if you download in advance.
In the Founding Fathers' Footsteps (iPhone): Tours of Paris focused on Americans visiting at the time of the American Revolution. See other Blue Lion Guides apps.
Visit the Louvre (app): Get info on a particular piece of art or collection.
Le Fooding : This website, written by Parisians, highlights the best, the trendiest, the most interesting, the cheapest, etc. restaurants, food trucks and bistros around town. Links to apps are available on the website.
Spotted by Locals: Art, exhibitions, restaurants, activities curated by locals.