Baidu Translate (百度翻译) Similar to Google Translate, C-E/E-C translations but more. OCR (an in-app purchase in Pleco), useful common phrases/expressions list, audio lookup, save your lookups. Android/iOS.
Chinese HSK it has several HSK levels with some useful grammar and vocabulary games, great for practice. It has also dictionary and special levels (Android)
ChineseSkill iOS and Android app that provides robust learning materials up to HSK4 and exercises in speaking, writing, listening and reading. It also has an internal flash card system that allows reviewing words and phrases in different ways. Available in 8 languages: English, Spanish, Portugese, Russian, Thai, Vietnamese, Japanese and Korean.
Hanping Chinese Dictionary Pro Android only. Includes handwriting recognition, native speaker audio, stroke animations, example sentences, idioms, HSK words as well as tools like realtime clipboard monitoring, homescreen widgets and soundboard. Multi-syllable audio, extra stroke animations and ABC dictionaries available as in-app purchases. Part of a suite of apps that include Hanping Camera (considered by many the best Chinese OCR app on smartphones) and Hanping Chinese Popup an innovative handle/cursor that floats over all apps allowing you to look up words much like in Browser Add-ons.
HelloChinese is an app for improving speaking skills based on speech recognition. The speaking exercises are very interesting.
HelloTalk Language Exchange App
This App has more than 10,000 native Chinese speakers seeking language exchange partners. It has text and voice exchange mode to count number texts and duration of talks between language partners.
PinyinBrowser This is a special web browser for iPhone and iPad that adds Pinyin or Bopomofo readings to Chinese web pages in real time while you browse.
Linqapp Connects you instantly to native Chinese speakers for language exchange or for answers to your questions about anything Chinese language/Chinese culture related. Post photos of characters for translation, ask for audio recordings on pronunciation, ask about a grammar structure, ask anything-- watch the answers roll in.
NiHao! (see demo) This iPad-only game (Commercial) is both a grammar book and a puzzle game. Users are given Chinese puzzle to unravel as they learn Chinese grammar.
Pinyiner Free Android app for pinyin annotation of copied text or book files
Pleco (iOS/Android) Flashcard/dictionary/text reader. Perhaps the most comprehensive dictionary/flashcard/OCR learning software available. A lot of different dictionaries available. Lots of customization available. Text-to-speech, importing text from the clipboard, and input by radical are all features. Free version provides a lot; in-app purchases provide features like looking up words via OCR using your phone's camera, handwriting input, and importing text files with on the fly popup translation.
Skritter is the best app for practice writing, very beautiful and responsive, web version is also great (iPhone & iPad, also with an Android beta)
Wise Talk (see demo) This free iPhone app lets users hear and read Mandarin, English, and Cantonese using familiar idioms. Connects with system dictionaries to let users lookup Chinese and English word definitions. An unlocked version includes over 4880 idioms.
Word Tracer - Learn Chinese is another great iPad app for practice writing. It lists characters by frequency ranks, stroke counts, radicals, pinyin, etc. and also lets you define your own custom lists of characters to tailor to your own learning experience.
HSK Locker This is a very efficient free Android app that helps with practicing or growing your Chinese vocabulary (based on the HSK 1-6 levels) at your own pace.
Beginner Chinese This is an effective Android app designed for beginners or enthusiasts as an introductory flashcard tool that uses the 'a-word-a-day' principle to slowly build your vocabulary from scratch.
Inkstone (see demo video) An Android app that teaches how to draw Chinese characters. Contains stroke suggestions, enforces proper stroke order and direction. Has a built-in database of Pinyin, meanings and compositions. Source code available.
TOFU Learn is for learning many languages including Chinese, and its app includes character writing practice (like Skritter). It's described as free, but it also has paid "pro" accounts.
Anki is a flashcard learning system that has many Chinese 'card decks' created by the community. Some decks have sound, pictures and pinyin for C-E, E-C and sound-E learning. Apps for Windows, Mac, Linux, iOS, Android and Web.