The following is a list of essential travel apps which make life much easier on the road. Apps that work offline are noted and are very important when WiFi or cell service is not available.
Maps.me maps (free, offline) - This is the most important app on my phone. Popular landmarks, hotels, restaurants, etc are already marked on the maps and the app tracks your current position whether you are online or offline. Maps obviously need to be downloaded in advance for offline access. Maps.me also works in China where Google Maps is blocked. Click here to go to the download page for Maps.me.
Google Maps (free, offline) - Works well offline but doesn't work in China and in general isn't as nimble as Maps.me. It is much better than maps.me for showing bus and subway routes and schedules when internet is available.
Booking.com (free) - Has the world's largest selection of online hotels. Many hotels give a 10% discount for frequent bookers known on the site as 'Genius' users. Only two bookings in a two year period are needed to automatically become a genius user and qualify for discounts and other perks. The other huge advantage of Booking.com is that the reservations are guaranteed and if the hotel doesn't have a room then you will be given an equivalent or better room at another hotel. Click Here to Book Hotels on Booking.com.
Podcast Addict (free, offline) - Essential for providing free listening material for those long plane, train, and bus rides. Thousands of podcasts are available for download.
Currency Plus currency converter (free, offline) - Outstanding tool to keep from getting ripped off when changing money in the street. It shows exactly what rate you are getting and can display multiple currencies at the same time.
Google Translator (free, offline) - Languages can be pre-loaded for offline use. The speech recognition is weak when there is a lot of ambient noise. Doesn't work in China.
Expedia.com (free) - Good app for booking flights mainly because it gives you 24 hours to cancel a flight for free if your plans change.
Trip.com (formerly CTrip, free) - Excellent for booking trains in China and flights in many other countries. For trains, the service charge as a percentage seems a little hefty sometimes. However the app is very useful to avoid language difficulties when buying tickets. Tickets still have to be picked up at the train station but the agent just has to be given a reservation number. For flights, the prices are very competitive compared to other services like Expedia.
ExpressVPN (free trial then monthly fee) - This is a must-have app in China to provide access to the many popular web sites such as Google, Facebook, Instagram, and YouTube that are blocked. The app is also useful outside of China for example if you need to do online banking and you don't want your bank to freak out because the login attempt is coming from a strange country. You can pick the country that you will appear to be connecting from.
Wikiloc (small annual fee for offline maps) - Has easy to follow GPS maps full of hiking and biking trails all around the world. Your location is shown on the trail map for easy offline navigation.
Wikiroutes (free) - Has bus route maps for many cities.
Pedometer Step Counter (free) - Throw away your battery sucking Fit Bit and start using this app to track your activity. It may be a little less accurate than a dedicated step counter but it is consistent which is the most important factor.
Google Voice (free) - Provides a free telephone number where you can be reached by phone or text message anywhere in the world. It can also be used as your WhatsApp telephone number.
Lightning QR code reader (free) - Useful for places where WiFi passwords and other important information are provided as QR codes.
Nice To Haves
Amazon Kindle (a few free books) - eBook provider and reader.
Audible (free trial then monthly fee) - Audio book provider and reader.