Some of my most recommended and favourited apps:
FlightStats - most recently downloaded app but not sure why I didn't download this earlier! You can look up your flights and it'll tell you the terminal, gate number, baggage carousel, and any delays!
Guides by Lonely Planet - itinerary planning app. You can download guides by city plus offline maps. Solid app if you're looking for things to do or eat.
Google Maps - OK this is any smartphone standard app. I use My Maps a lot so it's helpful to visually see all the places I have pinned on my phone.
Lyft - best app to get around. Click on the link to get a $10 credit
Google Trips - helps with itinerary planning. It's extremely limited though- I think they're still working on it but there's a lot of improvements they need to make. The high advantage they have is that it integrates with gMail so they have tickets on file but there's no feature to search certain restaurants or places like you would on Google Map so you're limited to starring places they feature.
TripIt - I'm currently in the market for a good travel journal app so I can record what I've done on my trips. This one seems OK so far, probably the better free ones in the market. It lets me record my activity by day and time, and I can add photos and notes, which is nice. The other really nice thing about it is that it integrates with my gMail so it automatically records tickets for concerts, shows, and flights that I have. However, I wish the mobile app was as good as the desktop web version, and I also wish it was more integrated with other apps like Google Maps so I can see all the places within that city I've gone to, or be able to star different places that I'd like to go to.
PriorityPass - keeps your membership card stored (although most lounges require your actual card) + tells you which lounges are in which airport with super specific info like terminal, nearby gates, hours and amenities.
AirBnB (link gives you $40 off your reservation)
Units Plus (unit converter because America doesn't use the metric system while everyone else does)
Skype - to make international calls + if you want to subscribe to an international callback number
flight price tracker apps