Really depends on where and how you plan to travel in China.

At least a Chinese/English English/Chinese translator that works offline. I use one called Hanping Chinese, it has a free version that I use, but also has the pro version if you want to spend money.

Plus I download the subway/ metro map for whatever city I'm going to visit. (Have to say visit because the village I call home barely uses buses much less a subway)

Also there's free versions of programs like Talk Chinese. The give you a handful of useful phrase that it can say for you. I use the Talk Japanese, German, etc at the bars here as a fun little ice breaker that interests many of the locals.

Aibang train app allows you to search national train timetables and train destinations. Great if you're planning to do a lot of travelling by train.

If you're in Beijing, the Beijing taxi guide is really useful if you don't speak a word of Chinese. It has a large database of restaurants, hotels etc. with addresses, so the driver just needs to take one quick look and know where you're going.

http://itunes.apple.com/us/app/beijing-taxi-guide/id287346119?mt=8