Below are some links to some freely available corpus linguistics/NLP tools:

AntConc

AntConc is a great free concordancing program written by Laurence Anthony.  Check it out here.

Coh-Metrix

Coh-Metrix is an advanced natural language processing tool that analyses a number of textual features related to cohesion, lexical sophistication, syntactic complexity, and text difficulty. Check it out here.

VocabProfile

VocabProfile is a great text analysis program with a free online interface. I find it to be particularly useful for quickly assessing learner text difficulty. The web interface is maintained by Tom Cobb and is based on the program Range, which was written by Heatley and Nation. Check it out here.