Last semester one of my colleagues, who is working on a linguistics PhD sent me a link for a concordancer that he found to be useful. I gave it a look, and it wasn't just a concordancer, there are several of them along with 22 other tools.
I have used, and was using today, the vocab profiler. It is great. Just type or paste in some text and get a full accounting of the vocabulary included. I am doing a presentation at a speech contest on Monday next week and used some of this data in preparation.
In the past I pasted in a movie script for a feature length film and generated a word list for my students that shows them all of the words in the film according to the frequency of their occurrence. It was also a great lesson, because it showed them that the individual words aren't so difficult, but that difficulty in understanding comes because they are not used to listening.
The site is the Compleat Lexical Tutor (not my spelling). Give it a look and enjoy!