I've just come across an interesting cooperative enterprise uniting clinical work from seven U.S. law schools ((Harvard, Stanford, Berkeley, University of San Francisco, University of Maine, George Washington, and Santa Clara University)) and the Electronic Frontier Foundation. Chilling Effects Clearinghouse has created a searchable database of cease and desist letters, a significant proportion of which are designed to bring a halt to legitimate online activity. You are invited to submit a C&D letter to the database; and the Chilling Effects team will annotate the letter, translating some of the legalese into intelligible prose. There is also a form to fill out that will help you build a DMCA "counter-notification."
In addition to explaining C&D letters, Chilling Effects issues periodic "Weather Reports," analysing the behaviour of companies with respect to freedom of expression in various sectors of the internet. Take a look at "Blue Jeans Cable Not Cowed by Monster's Roar" by Wendy Seltzer in which she discusses an absolutely delightful (to me) riposte to a Monster Cable cease and desist letter delivered to a person who turned out to be an experienced former litigation lawyer. A paragraph from his reply letter:
It may be that my inability to see the pragmatic value of settling frivolous claims is a deep character flaw, and I am sure a few of the insurance carriers for whom I have done work have seen it that way; but it is how I have done business for the last quarter-century and you are not going to change my mind. If you sue me, the case will go to judgment, and I will hold the court's attention upon the merits of your claims–or, to speak more precisely, the absence of merit from your claims–from start to finish. Not only am I unintimidated by litigation; I sometimes rather miss it.
Back a few years ago when I had my first and only cease and desist letter, I found the Chilling Effects database to be quite helpful.