“Never confuse your right to say what you believe with a right to never be disagreed with and ridiculed for saying what you believe.”

Source: The Friends of Voltaire (1906), Ch. 7 : Helvetius : The Contradiction, p. 199; because of quote marks around the original publication of these words, they are often attributed to Voltaire, though Hall was not actually quoting him but summarizing his attitude with the expression. The statement was widely popularized when misattributed to Voltaire as a "Quotable Quote" in Reader's Digest (June 1934), but in response to the misattribution, Hall had been quoted in Saturday Review (11 May 1935), p. 13, as stating: I did not mean to imply that Voltaire used these words verbatim and should be surprised if they are found in any of his works. They are rather a paraphrase of Voltaire's words in the Essay on Tolerance — "Think for yourselves and let others enjoy the privilege to do so too."
The paragraph in which the statement first appears reads:
Context: 'What a fuss about an omelette!' he had exclaimed when he heard of the burning. How abominably unjust to persecute a man for such an airy trifle as that! "I disapprove of what you say, but I will defend to the death your right to say it," was his attitude now.

Variants:
Monsieur l’abbé, I detest what you write, but I would give my life to make it possible for you to continue to write.
I wholly disapprove of what you say—and will defend to the death your right to say it.
Though these words are regularly attributed to Voltaire, they were first used by Evelyn Beatrice Hall, writing under the pseudonym of Stephen G Tallentyre in The Friends of Voltaire (1906), as a summation of Voltaire's beliefs on freedom of thought and expression. http://books.google.it/books?id=j3kGAQAAIAAJ&q=%22I+disapprove+of+what+you+say,+but+I+will+defend+to+the+death+your+right+to+say+it%22+intitle:%22The+Friends+of+Voltaire%22&dq=%22I+disapprove+of+what+you+say,+but+I+will+defend+to+the+death+your+right+to+say+it%22+intitle:%22The+Friends+of+Voltaire%22&hl=it&ei=6J3uTbDYKcLX8gOnkLGTBw&sa=X&oi=book_result&ct=result&resnum=1&ved=0CCsQ6AEwAA
Another possible source for the quote was proposed by Norbert Guterman, editor of "A Book of French Quotations," who noted a letter to M. le Riche (6 February 1770) in which Voltaire is quoted as saying: "Monsieur l'abbé, I detest what you write, but I would give my life to make it possible for you to continue to write" ("Monsieur l'abbé, je déteste ce que vous écrivez, mais je donnerai ma vie pour que vous puissiez continuer à écrire"). This remark, however, does not appear in the letter.
Misattributed

Source: City of Fallen Angels

http://www.huffingtonpost.com/2008/06/07/clinton-concession-speech_n_105842.html, Washington D.C., June 7, 2008
Presidential campaign (January 20, 2007 – 2008)

Source: The Six Rules of Maybe

Speaking to the rest of the Hollywood Ten during their preparation for testimony, in answer to a hypothetical prosecution ploy, "Do you believe in free speech for fascists?" From Odd Man Out: A Memoir of the Hollywood Ten by Edward Dmytryk (1996, Southern Illinois University Press, Carbondale, IL).