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. Laurence has a number of other helpful tools on his software page as well, so check them out.

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. It is 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 out Tom's other programs here as well.

​L2 Syntactic Complexity Analyzer

Xiaofei Lu has a number of tools on his website that calculate syntactic and lexical elements of texts. He also has a few tools for comparisons between Chinese and English.