I've been using TripIt for years and I find that it is a great place to keep all my logistical information in one place.  Confimation numbers, rates, dates reservations have been made, addresses of places I want to go, flight confirmation information at-hand.  I recently had a VERY late departure and TripIt actually alerted me to the fact I might be eligible for a partial refund from the airline!  It is also good for sharing a less-detailed version with your traveling companions.  The app is easy to use on the road which I found helpful as I didn't need to carry the pages of confirmation print-outs which I had been doing for most of my traveling life.  There's a pro version ($49 per year) which includes extras like real-time flight alerts, TSA wait times, and loyalty reward program updates.

One of the best ways to determine the "must see/ must do" things in any given destination is to look at Viator and see what tours and activities they offer the most of in any location.  Additionally, you can book any of their tours (or transfers).  When I have decided I want to do a certain type of tour or excursion which I won't be doing myself, I look at the highest rated offerings at Viator (keeping in mind the size of the group) and then shop around to make sure there aren't other, better, providers out there (google "best xyz tours in wherever."  It has been my experience that when a provider has excellent ratings and reviews (always read the reviews!) at Viator, it will be a good experience.

If you do not feel like planning your whole trip and do not already have a trusted travel advisor, check out Zicasso, which connects travelers with vetted, expert travel agents and tour operators who specialize in luxury and custom trips. They use a rigorous selection process to ensure these specialists are the "top 10%" in the industry, offering deep local knowledge and high-quality service to plan unique travel experiences for their clients. 

The Google Maps app is ubiquitous and essential whenever I travel.  If I'm going someplace remote, I will download an "offline" map; if I have devised a walking route, I will save it and then access it on that day; if I don't know how to get to the starting point of a tour or restaurant, I just let the map app tell me.  (Should you not be experienced in following directions in this app, be sure to hold your phone flat with the top facing forward.)

Ah, international travel.  It's great, but communicating with locals can be challenging if you don't speak the language.  I used the app recently and was able to have street signs and menus translated using the photo icon in the app.  It was pretty great (although not perfect).

With this free app, you input your customs declarations and passport info before you land and then use a special line at the arrival airport in the USA.  If you do not use Global Entry (or another Trusted Traveler Program), this can be a real time-saver.

Priority Pass airport lounges can be found at many airports around the world, and access to one is always better than spending airport time in the regular waiting areas.  You get food, drink, comfortable seating and wifi, and that can be worth a lot for some people.  While certain premium travel credit cards include Priority Pass access in their fees (like the American Express Platinum Card or the Chase Sapphire Reserve card), you can purchase membership on their website.  What's great about the app is that you do not need to carry (or find!) the actual card to get into the lounges (which has been a life-saver on more than one occasion).  Just a warning -- during busy travel times, these lounges can be full and have a waiting list which completely defeats their purpose, in my opinion.

When you are trying to sleep or relax, and ambient sounds (of screaming babies on a plane, for example) are not allowing you to find your relaxation zone, this app can.  I've used it on planes and in hotels...

For those of you who want to get out and take a nature walk or hike (internationally), this is a good app to have with you.  It helps you find the trail head and includes ratings (although I find their timing estimates to be for extremely fast hikers, fyi).  There is a paid version of the app which I don't have but hear is very much worth it.