Robin Good: VocabGrabber is a free web-based app that allows you to easily visualize all of the keywords contained in any text and to identify the most relevant ones at a glance.
VocabGrabber, a service from Thinkmap Visual Thesaurus, instantly calculates frequency, main subject categories covered, and can sort by relevance, alphabetical order and familiarity all of the words therein contained and save them for you in a word list.
"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."
Vocabgrabber creates also a visual thesaurus map related to any word you want to explore, provides definitions for any term highlighted and examples of how that word is ised inside the text.
Free to use.
Try it out now: http://www.visualthesaurus.com/vocabgrabber/