Apps for Android or Ios

DuoLingo includes Chinese in its large range of languages. It is the best app I have tried so far, having been right through Hello Chinese and a good way through ChineseSkill (both good), among others.  There is an initial assessment to find your starting level - a great feature for non-beginners, and one that most apps are lacking.  This will start you at the right point without having to go through the basic stuff yet again!  You can do the whole course for free, though there is an option to pay about £50 a year to go ad-free.  This also enables you to download lessons for use off-line.  There is also a web version at 
     www.duolingo.com/course/zh-CN/en/Learn-Chinese-Online
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Hello Chinese is a great app to get going with.  There is some useful stuff near the end, though it doesn't get quite so challenging as DuoLingo.  It features useful touch-screen exercises for learning to write the characters.  There is no Web version, but there is a website at  www.hellochinese.cc/
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Drops is an app with a great interface that concentrates on vocabulary rather than grammar - which is the currently favoured approach at HML (or at least, by me).  The free version is limited to five minutes a day, plus occasional time bonuses.  You have to upgrade and pay for extra time, but I am currently sticking to the free version for use as a quick daily supplement.