iPhone, iPod touch, iPad: IntellVocab, available in three different versions, helps you improve your vocabulary using an Artificial Intelligence engine based on MIT research. The app, which is free through to the end of June, personalises and adapts to your practice to make sure you really learn new words.If you're a logophile or want to boost your vocabulary for a competitive exam, IntelliVocab is a fun, almost addictive way to do so. The app quizzes you with multiple choice questions on word definitions, synonyms and usage. When you answer questions incorrectly, the AI makes a note of it and then asks you about those words again later, in different contexts (e.g. instead of asking you to choose the straight definition, it'll offer a fill-in-the-blank test).
In addition to the practice mode which goes on as long as you have time, there's a timed test you can take, very gratifying progress reports (with customisable deadline) and word list lookup. One nice feature is that the voice actors offer multiple pronunciations of words where applicable, so if you're used to hearing "abject", for example, a certain way (ab-jekt or ab-jekt), that won't throw you off.
Of the three versions, the Business Communications one seems the least useful. As far as I could tell, its 236-word dictionary is already in the GRE/GMAT and SAT versions, and includes less business-oriented words than you'd expect ("laddie" and "lacteal" are two examples of words you'd probably use only in very specific business situations). The GRE/GMAT and SAT apps have 694 and 695 words, respectively; the dictionaries seem to overlap a lot, so you can pick whichever you'd like.