A lot of these applications are more language-specific. As my main foreign languages are French and Spanish that is obviously my focus, but a lot of the reference materials and tools are available in a wide range of languages.
Language learning tools
Most of them are word-based only or of the phrase-book sort, so look out for grammar or exercises highlighting language structure as a real bonus.
Free Essentials (free)-full version is £4.99: Listen to vocabulary from different categories, use flashcards and spaced repetition, quizzes including an audio quiz and a basic translation feature.
24/7 Tutor lite (free)-£4.99 for more advanced versions like Spanish or French 101, 102, 103... German, Italian and Italian also available in the lite version: Work from different categories and practise with multiple choice, puzzles, writing-in exercises and flashcards. There are 2 categories concentrating on basic phrases and questions. All exercises are supported by audio files.
Odyssey Translator ( free) Phrase book with some emphasis on structure and supported by audio sound files.
World Nomad (free) is available for languages like French, Spanish and Chinese. The applications consist of an audio phrase book with a podcast. The aim is for the language to be used while travelling so there is not much explanation given about how the language works.
Mandarin Fast Track Lite (free): nice background information about the language, with audio, Chinese characters, pinyin, word practice including basic recording and playback facilities and quizzes. Full version is £5.49
Chinese for Munchkins (free): animals, numbers and memory games (full version £1.19)
Basic French for Dummies (£0.59): word/ phrases/dictionary/ flashcards/basics/common mistakes/ share what you are doing via Facebook or email and encourage your friend to answer back in French
WordPower Lite (free): word of the day in Spanish uploaded daily: Listen, read, record and playback for practice. (full version with 2000+ words costs £5.99) Available in other languages like French, Italian and Chinese.
Spanish WordTrainer free (full version is £1.19): Listen, repeat, test yourself, compete against yourself and others, save specific words in favourites for future use or practice, all supported with audio files.
Interpret and itranslate are both free multilingual translation tools, text-to-speech options can be bought as an extra for just over £1.
Memorize Words for Spanish Lite (free): flashcards for English and Spanish (French version also available), sound and spaced repetition, glossary and games to help memorize new words (full version costs £3.99 but if it works for your students, you could do your own spaced repetition podcasts using free recording software like Audacity)
Parlez Chinois (free)-a n excellent Application for French speakers wanting to learn Chinese with videos, listening exercises and lots of audio support
My Chinese library (free): phrase book with sound files and the facility to add phrases and compile your own phrase book. There is also a useful pronunciation guide about how to read pinyin.
Ilearn Chinese characters Lite (free) is a very interesting application introducing Chinese characters and their origin as well as the correct way to write them. It even lets you practise on the iphone side by side with a model. (a full version is not yet available)
Chinese word of the Day (free): a nice way to learn something every day with native speaker audio, pinyin and translation in English. Available as a free application for many other languages including Arabic, Tagalog, Korean, Latin, Polish, Japanese, Greek, Portuguese...
WordReference (free) French-English, Spanish-English, Italian-English dictionaries with verb tables and sound files
Les nombres (free) Writing numbers in French: just type in the number to check the spelling
Le Bescherelle (£0.59): a classic to find out about French verb in all tenses and moods
Le Petit Robert (Dixel): Monolingual French dictionary, a bargain at £5.49 as it includes common and proper nouns as well as some word games. Le Dictionnaire Larousse is another good alternative at £3.49 even if it does not include proper nouns.
Isinonimos (free) Spanish thesaurus
El conjugador (£0.59): very user-friendly application to check conjugated verbs in Spanish
SpanishDict (free) dictionary word games, phrase book, word of the day
WordWeb English dictionary and thesaurus (free) is good but as there is no audio attached to it, the WordWeb Audio English dictionary is a better alternative with British English accent.
Dictionary.com (free): dictionary, thesaurus, search, word of the day (with sound)
More to come still...
More to come still...