Frank Zappa sent a long, fascinating letter about the "Parental Music Resource Center" (a mid-80s music censorship outfit) elaborating on why he objects to censorship. The scanned letter, including its hand-addressed envelope, is available on Flickr. Link (Thanks, Burnt!)
Update: Paul sez, "This link has Frank Zappa's full testimony before the Senate Committee on Commerce, Science and Transportation, back in September 1985. It includes a great prepared statement of Frank's. You can also read the testimony of musicians John Denver, and Dee Snyder of Twisted Sister (God, I wish I'd been on a field trip in D.C. on THAT day.) In addition to the historic value of the testimony given (remember these were the hearings that led to the 'voluntary' labeling that is in widespread use today) there is also great comedic value in hearing, for instance, Jeff Ling of the PMRC (Parent's Music Resource Center, Tipper Gore's little hellspawn) read lyrics from the Mentor's song 'Golden Showers' before a panel of senators ('Bend up and smell my anal vapor. Your face is my toilet paper.') This hearing was masterfully remixed by Frank on the twelve-minute-plus 'Porn Wars' track on the release 'FZ Meets the Mothers of Prevention' to great effect."