VocabGrabber analyzes any text you're interested in, generating lists of the most useful vocabulary words and showing you how those words are used in context. Just copy text from a document and paste it into the box, and then click on the "Grab Vocabulary!" button. VocabGrabber will automatically create a list of vocabulary from your text, which you can then sort, filter, and save. Select any word on the list and you'll see a snapshot of the Visual Thesaurus map and definitions for that word, along with examples of the word in your text.
Dictionary.com is an app that has an integrated dictionary.