I've lived in China since August, and have to say that the apps listed below have made my life in China a whole lot easier! If you're going to China for more than a month, the apps below are worth a download. Some of them are likely to be helpful for shorter trips, too.
1) Express VPN- A lot of the internet sites I use daily are blocked in China, such as Twitter, Instagram, Google, and Facebook. The way to access them, of course, is by buying a VPN. For China, Express VPN came highly recommended. Truthfully, my friends who have Astrill are always complaining that they cannot connect. I bought a year's subscription to Express VPN for $99.95, and that allows me to use my VPN on two devices. Click here if you're interested in signing up with my referral link. By signing up through me, there is no extra cost to you, and we will both earn one free month of the service. For me, this was 1,000% worth the money, since I have a long stay in China. If you'll just be making a short trip, I can see how you would forego signing up.
2) Skype- This app is useful when I have to make calls to the States. I've asked around about where I can buy calling cards, but everyone just recommends adding credit to a Skype account in order to call home. I'm NOT a big Skype person, and prefer to stay connected to friends back home using different medians, but having this app came in handy for me when I had to sort stuff out with my bank.
3) We Chat- We Chat is HUGE in China. When I first arrived and asked about a text messaging plan, the guy that sold me my phone said, "Oh, we don't actually have a plan for text messaging because Chinese people just use We Chat." I've yet to meet a Chinese person who doesn't have a We Chat account, and it certainly proves helpful to get in contact with other expats, to write my Chinese co-teacher, and to stay in touch with my friends and family back home. I made a few of my closest friends download it, and my sister and I message each other through We Chat on a regular basis. The best thing about We Chat is that you can create group chats, so it helps me stay organized at school since I'm in various chats with the other teachers. It's also ideal for my social life, since I get invited to events and outings via We Chat groups. A friend of mine visited China for a few days, and it was useful to him because he could just access it by using his hotel's wifi, so it didn't cost him any extra money to contact others already in China.
4) Pleco- I'll readily admit that my knowledge of Chinese is pretty much non-existent, save a few words and phrases. This app has saved me many times because I just type in the English word, and it translates it into Chinese for me. Of course, usually I don't know how to pronounce the word that comes up, but luckily, with this app, I'm able to play it out loud. It just helps me out in order to find what I'm looking for in stores!
5) Airpocalypse- This app is actually one of my faves! Ever since the air pollution started affecting me and making me sick, I'm constantly checking this app to see what the air quality is in Shanghai. The pollution levels are updated frequently, so I always look at this app before going outside to see whether or not I need to wear my mask. It has a list of major Chinese cities on it, so I like seeing how Shanghai is doing in comparison. The app is also a bit snarky, which I find a bit entertaining. It will say things like, "Meh" or "Only in China is this normal" when describing the air. If being healthy is one of your priorities, you may find this app to be helpful.
Another app that a lot of expats in Shanghai use is Smart Shanghai. It's not comprehensive, but it has a list of places to go to around Shanghai. All you have to do is type the name of the place you're looking for and the address will come up in Chinese. This app has saved me a countless number of times when taking taxis. I'm also not going to complain about the map function!